There's a lot on info on Sempron in
anandtech's article.
Sempron is the newer of the two. It comes in socket A and socket 754 flavors.
Well... that's a start.
EDIT: I see your other question on A64. Are you building a new computer and need some input? I know I complain about the number of threads on this, but if you state a budget there are a bunch of people that come in here and will make suggestions on specific configurations for a given budget. If your budget matches a common one ($1000, $1500, $750, etc.) then there should even be some threads about it already with a ton of suggestions for you to look at.
If this is a gaming PC, A64 is what you want to shoot for if your budget will allow. If you're just doing 2d / light gaming / everyday internet and word processing, then sepron or AXP will give you all the performance you need and be easier on the budget.