R.M.
S.,
Based on Consumer Reports they say the firmer the better and that it is worth the extra few dollars to get one that has a wire frame (to keep the edges from dipping and possibly trapping baby in between) and that the most important thing is that the gap between the crib and the mattress be no more than two fingers..., but unless you buy it at the same place and time as your crib, this last part is hard to do.
We got ours at Target for $80 a few months ago. It's a Seally, I don't know the model number but it's a beige-y/gold tone w/o any cute design. The ones at Babies R Us were really expensive and although I would of liked to spend a bit less, we are really happy with it and my little one has never complained. Good luck and happy shopping.