If you only need two audio in/out have a look at the Steinberg UR22. We recorded our latest album with that interface, the pre's are excellent.....great litle unit for $150. The coolest thing is it has zero latency monitoring.

Sanderxpander
Always good to hear from multiple sides. Though your results perhaps say more about MOTU than about USB. So far I've used succesfully on USB:

Very good point.  I had to skip a number of very attractive I/O units with excellent customer feedback simply because they did not have the inputs and output flexibility I need for my studio.  I have a pic of the studio schematic below - but the MOTU and Focusrite series seemed to be the only ones which could handle the I/O count I needed at about the $1000 price point.  Here is a cut and paste from my schematic. (As you can see I have promiscuously financed the entire speaker making industry ... LOL!!! but gosh the power of the system is awe-striking..)

 
I route my surround sound buses solely to the Optical output, and compress/triple-route percussion buses through the optical output, the surround bus and to the Main bus feeding the Yamaha reference speakers and JBL subwoof. Then take my instrument tracks and triple route them to the specific applicable amp, and to the Main bus which hits the Yamaha/JBL Subwoof primary sound, and finally to the surround bus feeding the Optical Out. This provides for a strong, deep and present 'live band' sound in my studio.  I love it.
 
 

Actually that's why I initially got the focusrite, for zero latency. Funily enough I don't use it as much as I should nowadays (doesn't seem to make any perceptual difference esp when most of the time im using reverbs or amp sims when recording).

I echo some comments from above that you should not fear USB2 at all - particularly if you have a PC (at least somewhat) dedicated as a DAW.  I have a dedicated DAW (not used for common office stuff) running an RME UFX, which has both Firewire AND USB2 - RME recommends USB2 (and the unit uses USB2 by default) over FW as others have outlined above.
 
I've never had any throughput issues, even with large projects (75 tracks playign back while recording & live monitoring, etc).
My only caveat would be if you have a PC which serves as both a DAW and an office / standard home PC, and you have all kinds of peripherals connected via USB (printers, scanners, WLAN, etc etc etc).. I would exercise caution, as you'll have a significant chance of contention.
 
Hope that helps.
Russ

I just got the Scarlett 18i20, and it is a great interface.  The only difference between it and the Saffire pro 40 is that it's USB and not FW.  I can't believe how quiet the preamps are!
 
On the subject of USB, the only interface I would tell people to stay away from is the PreSonus AudioBox 1818vsl.  I had that interface, and really loved it the first few months I had it.  I then upgraded my computer and found that their drivers are incompatible with Intel USB 3.0 chipsets.  The scoop is that Intel USB 3.0 chipsets are not fully backwards compatible with USB 2.0 devices and don't support isochronous streming.  But it really has to do with the way that PreSonus wrote the VSL driver and now refuse to put out any updates for it.  PreSonus makes some great hardware, but their software support is terrible.
 

Thanks for the input everyone. This has really turned into a VERY helpful discussion. You all make excellent points. Too excellent in fact. :)

The truth is that it all boils down to what works best for your particular system and preference. It will be different for all. So, I have to say I now feel much more educated after digesting all of your comments...however, I also feel quite dumb about my confusion over which format to go with. Haha.

Good points interjected here! I am very similar in that I rarely run more that two audio inputs simultaneously. Russ' point above is about the only thing I would worry for with USB (other than brand), and you have already addressed that. For me, I do (very much so) use this machine for everything, and leave things connected, so the "dedicated and unused FW port" became an attractive option as I upgraded the audio interface (the heart of any DAW). When the mood strikes to record, I can simply turn it on, and off I go.
 
I think the "Focusrite" feedback zeroed me in on brand quickly... I have yet to see anyone "bad mouth" them, and now that I own the interface, I can confirm the "solid as a rock" (and heavy as one too!). The "MixControl" software it comes with is incredible and once that is understood (specifically routing), the rest has been simply "Have an idea, turn in on, pop open MixControl/X3 and have at it."
 
I love TStranger's schematic! That is so awesome and thank you for sharing.
 
As I tailored this system, I had a simpler idea in mind (sort of corny in a way, but important to me)... I wanted to be able to record an idea without losing it in "under 2 minutes" from a powered down condition, and have achieved that 100%.
 
 

Don't feel dumb. We are faced with a staggering array of options, and you posted a valid question.
Recently, I was faced with getting a USB interface. I was less than thrilled with the proposition, because USB interfaces I had been exposed to in the past had dismal low latency performance.
For the past several years, I had been using a PCI interface, which smoked the USB options at the time. But my trusty  XP PC died, and I was faced with making it work with a newer, but still dated Vista laptop. High track count has more to do with the CPU and hard drive, so that was going to be what it was. But I rely heavily on soft synths (triggered live via controllers) and amp sims, so low latency performance was an absolute must. And I have limited faith in USB bus power. 500 ma only goes so far.
I gambled on the AKAI EIE PRO because it had 3 USB inputs, 1 MIDI, and 4 audio inputs (w/48v phantom power available). This thing sucks some power. 6v 3a. USB can't touch that.
When I roll from direct monitoring to DAW output, there is no perceptible increase in latency.
The point being that with today's state of the art, a USB2 device and a current computer should perform quite well. 5 years ago, I was skeptical of USB. Today, I an rather impressed.

The truth is there will be far less confusion in a year :)

and Win7pro won't be top dawg either...?