I've been using a pair of Studio Projects C4 mics for recording classical guitar.  They were pretty much my first entry into SDCs and the set has 3 different capsules, for Omni, Cardiod, and Hypercardiod. They don't do too bad, but I'm trying to get a slightly quieter and better sounding overall signal chain for recording classical guitar and some steel-string as well.
 
After doing some reading up on specs and reviews I'm considering a set of Shure KSM 137 Cardiod mics to replace them.  They seem to be a little quieter and might be better sounding.  Anybody have experience with these mics have any advice?  Do you think they would be much of a step up in overall quality from the C4 set?  I'd hate to spend the money for a couple of decibels of better noise specs if they don't also improve the sound a bit, but if they are a much better all-around mic I might be due for an upgrade.

I haven't used the 137 myself, but I have such high confidence in Shure microphones in general that I'd have no reluctance to buy one or two. I've long wanted a pair of SM81s, but based on what I've read in recent years, the KSM137 would be my first choice today if I were buying an SDC for acoustic guitar. It's reportedly very flat with good transient response. You might also consider the KSM141, which is the same microphone but with an omnidirectional option, which would allow you to use M/S recording on your guitar.
 

personal observations only!!!!!!
 
I dislike the SM-81, I find it too harsh. For reference, I'm not a huge fan of the AKG C-414 either, and I have one of the older ones, which are supposed to be pretty cool by most accounts.
 
The thing is, I've only ever owned one microphone for which I never found a good use. I do use the C-414 from time to time. I just used it on an acoustic guitar (small body, Rosewood & Spruce) track where my usual go-to microphones (TLM-193 or Dragonfly) just didn't sound right in the mix.
 
I have used SM-81s on cymbals and hand percussion in the past, but there are so many other SDCs that I don't own a pair.
 
My fantasy SDCs would be a pair of KM-84s (not the KM-184s, they do sound different) and a pair of Schoeps CMC6s with at least the hyper-cadriod capsules. No, they don't sound similar, but they both do magical things with acoustic guitars!

So lets hope that puts my personal tastes in perspective...
 
My go-to SDC for nylon string guitars is a pair of Earthworks SR-77s. I love the detail that they capture.
 
My go-to SDC for steel string guitars is either an AKG C-451 with the cardiod capsule, or an A-T 4031, but sometimes I'll use one of the SR-77s. I seldom use a pair of SDCs on steel string guitars, preferring instead one SDC and one LDC.
 
I have one guitar, a 1977 Martin D-18, which is nearly impossible to record. For this one I use a TEAC PE-120 that I modified for phantom power. For whatever reason, this microphone just likes this guitar, or vica-versa. It's an oddball microphone for sure (it was also sold by Nakamichi with a different model number.)

The Shure KSM family is wonderful, and I plan to add a pair of the SDCs to my tool kit one of these days. They are probably about as close as you can get to an all-around microphone. They don't sound like anything really, and if you have to start somewhere I guess that's as good a place as any.
 
The other microphone I'd consider in that price range is the Michael Joly MJE-384. I've only heard recordings made with them, but I am really impressed. The pair is around $700, while I think a pair of the KSM-137s is about $50 less. The difference being the Shure is pretty darned neutral, while the MJE has character, not unlike my fantasy KM-84. No, it does not sound like a KM-84 sadly<G>!
 
There is also the KSM-141, which is a multi-pattern SDC, a strange beast indeed. I used a pair of them only once, and I ended up using them as cardiods, but it was fun, and interesting, to be able to change the pattern on the fly. I think they run about $100 more than the 137s, and if you don't have a room that will benefit from an omni pattern then I'd save the money.
 
Which brings up an important thought - while microphones and preamplifiers are important, unless you are using a hyper-cardiod capsule at a very close distance the space in which you are recording is just as important. So too is the instrument you are recording.
 
And of course the space is part of the recording... if I had a great sounding space I'd probably never use a directional microphone again (for solo recordings anyway). But like many here, I suspect, my space is far from great, and so I use directional microphones to mitigate the negative aspects.

It is a big, jumbled up mess really, composition, performance, microphone selection AND placement, preamplifier, and converter (these days) area all important, and while I think that order is probably a good one in terms of priorities, the fact is they are so intertwined that you can't really pick one - I guess one could argue that a great recording of a great performance of a so-so composition wouldn't be much, but when it comes down to the composition most of us like what we wrote or we wouldn't be recording it<G>!
 
Which is a very round-about way of saying two things:
1) you really can't go wrong with the KSM-137, the entire KSM family is pretty darned good!
 
2) there is so much involved in recording, and especially recording a guitar (can you tell I'm a guitarist?) that getting too hung up on any specific link in the chain could be a distraction.
 
Hope that helps in some way...

FWIW, The newer C-414 XLS sounds a whole lot better (to me) than the older C-414 BULS on acoustic guitar (and just about anything else for that matter).     I have both...
 
In a perfect world, I'd want a pair of KM-84 SDCs running thru GML mic preamps.
That's as good as it gets... (and the cost reflects it)

The KSM 137 and 141 both get lots of good user feedback. I am sure they are "safe bets". Will you love them?
 
I had a pair of C414b/uls's, C480 CK61's (cardioid), SM81's and loved each of them on certain guitars and not on others.  I owned a U87ai and used it with one SDC on acoustic guitars, too. I loved the SM81's on my Fender Springhill maple jumbo and a sweet Taylor 514. I didn't like them as much on my '79 D28, nor my Galloup LG1 (a big loud rosewood cannon!). Those mics and guitars are all gone, though I have replaced one of the AKG C480b/uls-CK61's (Omni and hypercardioid caps are available).
    I also now have a Mojave Audio MA101fet (which comes with Omni and Cardioid caps)and am about to decide (or see if I can decide)which one to get a 2nd one of, as I do want a pair of SDC's.
   My 1987 Takamine C132s is in limbo having been at my repairman's shop for months, while he decides whether to retire or not?!?!? My options for a good recording space are very limited, so I hope to test them out at 2 local studios, in better sounding, bigger rooms.
    When I had my big pile of mics(37 including the above), I also didn't record the Takamine nylon, as I preferred an Alvarez Artist Fusion (A/E cutaway nylon)that had lots of work done to improve it, and so I don't know which of these current mics will be better for the Tak. I also just got a Taylor GS5 and have yet to record it, so it will be an interesting spring, while I get to experiment.
    So, my opinion is try to rent/borrow/test which ever ones you're interested in, first. It is less about money, than sound. It really does come down to your ears, your taste, your opinion and also, your guitars, spaces/room(s), and preamps, too.
Michael

Thanks everyone.  Yes, the KM-84s would be nice, but out of my budget.  I found a matched set of 137s in new condition (I hope) for a pretty decent price and snagged them.  Wish I had thought about checking out rentals like was mentioned here, but I think they will probably be a decent mic for me from the descriptions I've read.
 
Spent most of the day recording with my new mic pre today, a Focusrite ISA Two.  It took a little while to find the sweet spots, but I think it will work out OK.  I did acoustic and classical guitar with my C4s, and did a few vocal tracks with an SM7b.  Once I got it dialed in I was pretty happy with how things sounded.

The 137's and 141's are some of the best SDC's you can get for anywhere near the money.   Schoeps-like sound and a super fast transient response.  Leagues better than Studio Projects.  It is a good choice for a wide range of sources, including classical guitar.

That's great to hear Daniel, thanks.  The 137s shipped today, so I hope I'll have them in time for the weekend.  Really looking forward to checking them out.

Just to follow up on my microphone chase:  I received the set of 137s a couple of days ago and finally had time to do some recording today.  Much, much nicer.  More clean output and a much more balanced sound.  The highs are much smoother, still there, just no harshness to them.  I did some decent recordings with the C4s in the past, but these K137s are worth the money. Very happy with the quality of tone... now I just have to work on my playing.

Jim RoseberryFWIW, The newer C-414 XLS sounds a whole lot better (to me) than the older C-414 BULS on acoustic guitar (and just about anything else for that matter).     I have both...

Mine is actually pre-BULS, it is just a C-414. I will have to try the new one now (ouch!)

Jim RoseberryIn a perfect world, I'd want a pair of KM-84 SDCs running thru GML mic preamps.
That's as good as it gets... (and the cost reflects it)

Well yes, that would be an absolutely LOVELY input chain!!! You sir have good taste!!