From the choices you give I'd go for the Firewire 410. Deffinitely go for Firewire - more stability and more bandwidth.
If you insist on USB, you can get the M-Audio OmniStudio for $189. For that you'll get 4 ins and 4 outs. This is what I have and I'm happy with it, but if I was purchasing now, I would certainly go Firewire.
I wouldn't do anything less than 4 ins. This will allow you to simultaneously record your stereo Tyros, plus your vocal and a guitar onto 4 different tracks. You can later tweak and add effects. Very cool.
In addition on the 410, unlike your first choice, you get 2 preamps with phantom power. A must if you want the warmth and power of a condensor mic.
The 410 would double up as a great recording soundcard to be used with any recording software.
Chony
[This message has been edited by chony (edited 05-20-2005).]