I would stay away from s/pdif, I have it on the back of my 002R and it only supports 2 channels. s/pdif was something Sony and phillips came up with and its kind of limiting IMHO. Fire wire is faster than USB but USB 2 is faster than firewire, The big advantage firewire has is that it can be daisy chained a lot.
That being said, Pro tools comes in 4 different flavors
1. Pro Tools Free- 8 tracks and really not worth it being that you have to be running windows ME for it to work.
2. Pro Tools M Powered- Works with some M audio interfaces But you must buy the software and the interface separately, also M Powered does not come with a lot of stuff LE does (reason adapted, live 5 lite, etc).
3. Pro Tools LE- if your going in to get this one spend a little more and get the factory bundle (extra plug-ins). You get LE free with any version of the M Box or 003 (or 002 off the bay).
All of those are native systems, meaning how powerful they are depends entirely on your computer. LE and M Powered are both 32 tracks but can be upgraded to 48. Then there is the mack daddy:
Pro Tools HD (some times referred to as TDM)-This thing runs on it's own separate card and is big bucks. But if you want to know it comes in 3 different levels of power; Accel 1 thru Accel 3.
Check out this link for more info http://www.sweetwater.com/shop/pro-t...ools_guide.php
As I have said before I run Pro Tools on a mac (actually I have 2 Pro Tools mac set ups). The M Box2 (comes with LE) at $450 Should take care of your needs if you want to go Pro Tools (I suggest M Box2 Factory), but if you want to have the ability to add up to 16 pre amps of your choosing and record straight to the track not just mixer to a stereo track go with a 003 or 003R. The 003 and 003R have 4 XLR ins with pre amps built in, 4 1/4" ins, 8 channels of ADAT lightpipe, and 2 channels s/pdif (you have to choose between ADAT or s/pdif, both won't work at the same time). If you go this rout you can have pretty much any pre amps you want and don't need a board unless you want to record more than 16 tracks at a time.
Now an important thing to realize is cuebass is just as good and way cheaper. I don't really know cuebase that well but I bet Rip could tell you all about it.