USB vs FW vs TB... oh the decisions.....
 
Just to chime in, if you work in 24 bit and 44K USB2 should be fine for tracking a few audio tracks at a time....
 
However, if you want to move up the audio stream to higher sample rates and bit depths, you might want to look for something with more bandwidth.
 
I am not a tech geek so I don't know the whys but I do know that my DAW started to choke with USB2 at higher resolutions, tracking with 4 or 5 mics at once and trying to monitor in real time within a SONAR project.... spitter spatter, delay delay...  same system with a TB audio card connection (APOLLO) and no issues running in WINDOWS 8.0 with very low settings for audio buffers....  funny thing was though, that the CPU couldn't keep up with the DATA STREAMS and I had to set the number of threads for SONAR to be limited at 5 so the other 3 were available of the OS for housekeeping (and moving all this data around quickly).  Now my system screams but  only because I track in 24bit 96K but like I said for a couple of tracks at lower resolution, USB2 is fine with a modern DAW computer (quad core running x64).
 
Jim