this is a good reference for usability
http://www.pro-tools-expert.com/pro-...dware-checker/
Sweetwater use to do a nice straight forward comparison chart but maybe that was only in the catalog but also that is hard to keep on top of
so you want 4 mic inputs
maybe some extra inputs for later
do you want a headphone jack
do you want speaker volume control?
do you want USB or Firewire interface? or Thunderbolt?