I think it's software bundle and 5.1 sound vs. 7.1 sound, and maybe SPDIF output, since those are the things creative has been fiddling with in an attempt to artificially create a product range. The core technology is the same in each generation of their cards now, so it's mostly minor differences in how many channels, the software, and what ports they put on the backplate. That way they can charge a little less for their "value" cards and a whole lot more for their "platinum" cards (or whatever it is they call their premium line).
Your best bet is to look at the card specs and see if it matches your system. Do you have digital speakers? If so, you need a digital SPDIF output that matches your speakers (optical or copper). 5.1 speaker set but not digital? Then you want 5.1 speaker output jacks. Do you want a full theater setup? Then you may want 7.1 output. Maybe one or the other has or doesn't have a joystick port. Do you want firewire on the back or on a front mounted plate? Decide what you want and then get the cheapest card that plugs into your other hardware and does what you want, because they'll all sound pretty much the same and use the same drivers.
The included software bundle... If you don't have anything else to use to edit and play sound/video files, then look closely at the bundle but there are reasonably priced alternatives that work better and may be less intrusive than the creative apps that tend to take over your computer.
I think creative.com has a card feature comparison tool that will let you put the card specs side by side so you can see the differences. If you're going for the value card, you might also consider an OEM version of their regular audigy2. It won't have any extra cables or any software, but it might be cheaper or have different connection options.
