You should be ok with USB. I've had and used multiple peripherals over the past years that were USB and Firewire (printers, audio interfaces, etc) and can say, honestly that real-world performance is a wash. Yes, FW can be faster, but in my experience, USB is far more stable 100% of the time - but either are acceptable for our DAW uses IMHO.
 
Since I use a laptop exclusively for SONAR and that laptop doesn't have a FW port, I went with USB this go round (Scarlett 18i6) and have had absolutely NO issues with it thus far. And that's with my interface, a keyboard (qwerty), a mouse, a cooling lap desk (additional cooling fans under the laptop), Lexicon MX300 and my DigiTech RP355 multi-effects pedal ALL plugged in to USB and fired up at the same time. As long as I tell SONAR to not use the Lexicon's MIDI ports, and not to use the DigiTech's audio ports, they all just hum along, happy as can be, not upsetting each other.
 
Another reason I went Focusrite though, and to your point, is that I use my Lexicon MX300 as a hardware plugin in SONAR and have to use its S/PDIF I/O to do so. The 18i6 was one of the cheaper (yet fuller featured) interfaces with a S/PDIF I/O that is a USB connection that was available when I started building up this system a couple of years ago.

Well I got a defective one, or at least I hope so. Thing hooked up and worked fine, then after a bit, stopped talking to the software which claims nothing is hooked up. Power cycling it doesn't fix the problem, just has the green and red lights for each of the two channels turn on. So back it goes. Hopefully the next one will work better.

SubSonic - I'm right there with you. All kinds of stuff hooked up with USB and everything works 100 percent of the time.

I have the Scarlett 18i6 and it has plenty of excellent aspects. Totally satisfied with it.

I just went from an Edirol M-16DX to a Focusrite Scarlett 18i20 and I've been very happy with it and think USB is they way to go since firewire seems to be on the way out.  I have a lot of stuff hooked up to my USB ports and everything works great.

I've been running the Focusrite Scarlett 18i20 for a while now and am very happy with the performance.  Absolutely no problems to date.

I may be late to the game, but my opinion is to go with USB since it's more future-proof than Firewire.
 
However--and this is a BIG however--you will soon realize that not all Focusrite interfaces of similar variety are created equal. Even the 18i6 (USB) is different than the Pro 24 (Firewire)! You would think that they were the same except for the computer connection, right?  Nope!  Therefore, when choosing one or the other, you must be very mindful of the inputs vs. outputs; one may work for your input/output needs, and it may just be the Firewire one! <facepalm>
 
For Example:
18i6: Ins: Analog (8), ADAT (8), SPDIF (2) - Total 18
18i6: Outs: Analog (2), SPDIF (2), Headphone (2) - Total 6
 
Pro24: Ins: Analog (4), ADAT (8), SPDIF (2), "Loopback" (2) - Total 16
Pro24: Outs: Analog (6), SPDIF (2) - Total 8
 
Tricky Brits! As you can see, the Pro24 (the Firewire one) gives much more routing options. Those outputs come in handy if you have outboard effects gear.  That that loopback feature?  Very handy; almost required. In the Pro 24, it is most annoying that the headphone jack is tied to Outputs 3/4 (whereas the the 18i6 has them separate), which is fine for most cases and gives you the luxury of having a dedicated volume knob, but this will create nasty feedback loops if you decide to use this as an output to any outboard gear as an effects bus! So a drawback to this is that essentially, unless you're using a separate mixer, you inherently lose the ability to use Outputs 3/4! <facepalm again>

I retired an M-Audio Firewire Solo and bought 2 Focusrite Scarlett's; a 8i6 and an 2i4.  Both units have worked great for me, the latter I use more as a mobile unit.  If Firewire was faster, I really have n't noticed it on either of these 2 units.  I have noticed much more of a difference in computer processors than USB vs. Firewire.