All else being equal, USB is the way to go.  FW is fading and as it gets older there will be more and more problems.  I doubt if anybody is writing new drivers.  The last FW unit driver upgrade I know of is for my home TC Electronic Konnekt and that was over a year ago.  TC has no USB units, while most companies are switching to it.  Note that I have no problems w/ FW, but I bought my unit many moons ago.
 
If you can afford it, the Tascam uh-7000 is the best sounding in- class USB stereo unit.  Superb converters and pro preamps.  Latency is high, no midi and the soft mixer is pretty low rent, but for sound you can't beat it w/ a red-headed rented mule.  I use mine via AES to the TCK 48 when recording.
 
If I was looking for a new unit I'd probably go w/ TASCAM too.  Their new USB units are cheap, Craig likes 'em and the USB drivers are done in house, not rented out like the 7000.  I'd still check the latency on them, but otherwise they look like winners and should be well tested with SONAR since Gibson owns both.  RME is good, pro hardware with superb drivers but more expensive.  A step up is the new lynx PCI units, but of  course you need a desk top for that.  But it is the best of all worlds - low latency, great conversion and  a decent price @ $1000.  Lots of people still use their older lynx units for pro work, and these spec better (tho I haven't heard them myself).  If you just want great, go for a Burl and Focusrite Red, starting about $7500.
 
When asking about equipment it is always nice to put out a price range.  People will go for all price ranges even if you do mention it, but at least some restraint is shown if you do.
 
@

Definitely looking at the $200-400 range, but I'd go higher if necessary. I don't need super low latency, but it needs to be pretty decent. Otherwise, trying to play a synth track to a recorded track can be a little annoying. I don't remember what the latency was on my old Emu card, but I seem to recall it was really good. It's been too long and my memory is shot.  :)

The Steinberg UR-22 is currently 150.00. I paid 200.00 a year ago. Works perfectly for me. As for the sound, I was under the impression that all professional sound cards sound alike. So I'm confused when people say that expensive ones sound better. If it's all digital, how is this possible?

It is mostly in the analogue side of the things - there are only a couple of companies that make the chips so everybody uses the same ones. But there is a difference.  Of course, once it is digital, there is no difference.  But coming in and going out ....  Not quite as much difference these days, but I can hear it between the interfaces I've used here at home (presonus, TC Electronic).  You can get good results from just about any interface today, but better equipment makes it easier and quicker as well as sounding that little bit better.
 
I don't know what instrument you play, but there is a difference between guitars, so they say, tho they all have the same # of strings.  There is a smaller difference in preamps, tho many prefer transformer-based.  And there is always the car metaphor - a jag and a versa will get you from here to there fine but the jag will drive and ride better, has better acceleration, etc. etc.  And while a jag loses a lot more value since it costs 6 x as much, your $200 interface won't even be worth trade bait in a few years.   A Lynx etc. also lose that much, but you can still sell it for $800 in 3 years.
 
Neiby, your best bet is to find a retailer that will let you "try before buy." I am able to use the Tascam while recording, but I ain't much of a player.  If the latency (about 10 ms on my system which I haven't tweaked for usb) is too much, return it.  And yea, the Steinberg is a good unit, built by Yamaha.
 
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It looks like there are quite a few units in the $200-400 range. The Focusrite Scarlett 6i6 looks pretty good, but I think it requires their Mix Control software, which I found to be a little confusing. After looking at some reviews on Amazon, the Steinberg UR22 seems to be a great choice. I'll do some more research and read some more reviews, but that looks like it might be a frontrunner for me. I just want simplicity and low latency. Nothing else fancy is required. 
 
The Focusrite does come with several plugins, but I'll have to do some digging to find out if they're all that useful or not.

https://youtu.be/M1avPHhiGw8

This walkthrough of MixControl is well done. If just tackling that UI without researching it, it can seem overly complex.
 
Edit: Inserted that link from my phone and only showed up as text.

Sweet! If I learn Mix Control, the Focusrite might be a better option. I wish the unit I had now wasn't so busted. The noise is ridiculous. It's almost like it's picking up noise from the environment. Maybe even from the computer, which would suck. lol  I don't know what else to do with it, so I might just trash it and start over with something new.

neiby
Black Lion Audio does mods to Focusrite stuff to upgrade the preamps and outputs etc, so you could check into having your unit "improved". Their mods are discounted for April, too.
Michael

Sorry if this question is silly, but are you sure that what you have is not a ground loop problem?

JoseC.
Sorry if this question is silly, but are you sure that what you have is not a ground loop problem?

Definitely not a silly question. I have noise with nothing connected except my monitors. So, PC connected to Pro 14 via FireWire, Pro 14 connected to powered monitors. I don't have the power adapter connected because I (perhaps wrongfully) thought that FireWire would power the box, which is appears to do. I was just thinking of connecting the power adapter to see if the behavior changes. I can't remember if I tried that last time I was fighting with it.