I'm not that up on sound cards these days, so hopefully someone who is will chime in (I'd like to learn, too). I would normally just go by the customer reviews of the cards to make up my mind on which would be a good one to get.
As for 7.1 versus 5.1, I think that just depends on if you want to run a 7.1 speaker system. I don't think that it would hurt to get the 7.1 card with a 5.1 speaker system, as you'd just have extra capability.