When I look for a sound device I have to consider the following (roughly in order of importance)
1. COMPATIBILITY!! Check ONLINE FORUMS BEFORE YOU BUY! I can't stress how important it is to ensure that your audio device will in fact properly function with the DAW and other software you are using. Check first. Just because something says ASIO compliant for example, doesn't mean it doesn't behave badly with your software. Before you plunk down $300 see if someone else has tried it first...
2. Enough Inputs and Outputs? (also phantom power support?) SPDIF doesn't count either - but some devices (such as the M-Audio FastTrack Pro) have an extra input and output, but they are SPDIF only! - useless, in many environments. Read the fine print

Turns out I can do better w/ an ancient a SB Live! card than a fasttrack pro in terms of # of IO ports - I use a headphone-only rehearse out for live stuff so the extra out is critical (for me at least). A fast track pro won't work for a guy in my shoes. Make sure you have enough "true" IO ports...
3. The bus interface device isn't incredibly important in my opinion. I prefer USB3 when possible because of the flexibility and availability of USB on many devices... Firewire is okay too, but it only really stands out as great if you have a mac studio setup.. of course external over PCI is nice, but you can't use it with a laptop...The speed of the interface doesn't matter too much, because every device is going to be fast enough to support its IO - BTW usb3 rates faster than firewire anyways - not that I care, but anybody that screams about bus performance is selling something, IMO..
All in all, I like the m-audio product line, but I can't recommend the FastTrack Pro, because it doesn't have enough analog IO...Get something with more in's and out's I don't really think 2x2 is enough, personally - unless you have SPDIF as well in which case I think it is 4x4...