Friday I am going to record a live gig - just 3 piece band. I don't have one of those flat profile mics to lay flat against the guitar amp grill. I do have a few large diaphram condensors and small - pencil mics. Which would be better for this purpose.
 
Thanks

First make sure the sound coming out of the amp is good. I would use a dynamic mic such as a Shure SM57 up close to the speaker and toward the edge of the driver looking inward at an angle. It is well known proven combination that you will get a good quality signal that will sound good later in the mix.

I'd try the LDC first. 
 
Right now I'm into ribbons for guitar amps.  Smoother than a 57.  But I wouldn't lay it against the grill.
 
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There's an interesting article in this months SOS by Mike Senior where he used 7 mic's to record a guitar overdub and then started playing around with different fader levels/phase etc to bring out different aspects of the guitar tone.
 
Remember, if you aren't close mic-ing, the room will play a big part in the recorded tone. The further away the mic(s) is/are, the more the room will come into play.

AT - the ribbon you using - is it a figure 8 pattern ribbon?

Yea, a figure 8.  MXL's 144.  You can get cheaper ribbons and I didn't spend much on it, but it works fine for work here at home.  Not a Coles or nothing, but it sounds closer to that than a 57.  I record the amp in a small room, loudly.  And, of course, it may be the big tube fender my guitarist has been bringing over.  But the ribbon does a good job of capturing the sound.  More like the guitars I heard from my record player in my mis spent youth.  Smooth, not as sharp.
 
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Wow AT what coincidence - that is the same ribbon I have! In order to get a strong enough signal from it I bought a "Cloud Lifter" pre which works great. It does record well when I do mid-side method on acoustic guitar. I never did think about using it for guitar amp --- but I am thinking that for a live gig, this might pic up way too much room. Which way do you orient it towards the grill.......

Ribbon mics on guitar amps are good and can sound great except they need to be a little further away hence the room factor or spill factor. This may not work so well at a loud live gig.
 
At a live gig a dynamic up close to the edge of the speaker cone pointing in on an angle toward the centre is going to just work for you no matter what. Think live sound micing up guitar cabs. You can do it the live way. There are smoother sounding dynamics too such as the Shure PL75L which I sometimes use on guitar cabs. The most important word in the OP is the word 'live' in the first sentence. You would not be using ribbon mics or LDC's on a live gig, or I wouldn't anyway. And even in the studio slight distances away the LDC sometimes does not sound as good as a Shure 57 does up close. Doing both in the studio is cool and definitely works as you can blend and mix them etc for a wider variety of tones.
 
If the guitarist is really good and the guitar and amp sound killer you will get a good sound no matter what. The playing should overpower anything. Why not make it easier on yourself and track that cab in a noisy live environment in such a way that you will have a really nice isolated and great sounding guitar track that you will need to do very little to later in the mix. (Any chances of recording his direct sound out of the guitar for possible guitar sim later approach)
 
I have just finished an EP here in my studio and the guitarist used a boutique amp custom built along the lines of a Fender twin. I tried lots of things but ended up using the PL75L on the edge of the cone. Result: killer guitar tracks that sit perfectly in the mix. But the guitarist was killer though, made all the difference! Maybe record the cab in the way you would like but do the 57 near the edge close in as a backup and you will be pleased you had both.
 

It does take some umph to drive them - fortunately I've got several nice preamps.  As Jeff says, I don't think it would be good for live work w/ spill from the rear.  The sound guy won't love you.
 
I back it off a bit - a foot or less (what is that in centimeters?), off to the side of the cone and angled in a bit.  I'm not a gutarist but I should be more picky, but the end result is nice.  The rear of the 8 hasn't been a problem (YET!).  Smooth is the word, tho the gutiar is playing gnarly rock and roll.  I actually think the gold standard for the songs we have been doing is the Rolling stones c. Beggar's Banquet etc.   the sound I'm recording takes to LP & HP filters like a duck to water.  The guitars fit in very nicely to the soundscape, gnarly, frequency restrained yet still sounding full.  I might start mixing that w/ a more jagged 57 or other dynamic sound, just to see - or hear in this case.
 
I haven't tried it on acoustic.  I plan on doing the next series of songs here at home w/ the vocalist through it, too.  I tried one female, but it was a delicate, whispy part and couldn't get enough gain w/o noise.

Thanks -- we ended up micing the amp, a little off center with a dynamic mic  and it came out good ! He uses an old-school Vox amp (not sure which model - but pretty small), and the "grittiness" was about just right. This is the first time I used a snake-splitter - so the cables coming to my mixer - then interface gave me a completely seperate signal from the band mixer / PA - super quiet and the crowd noise was very minimal.... On boosting the Ribbon for the vocals I would really considser that Cloudlifter. The neat thing about it is, it runs off of phantom power, but does not transfer the phantom to the ribbon and damage it.