Which is better, AMD Athlon or Sempron, and stats of each?
Per MHz they're the same.
Per cache, they're the same except Bartons are better,
doubled L2.
Per $, Athlon is usually a better buy.
If you're overclocking and don't care about the larger L2
cache of the Barton, you might still care about the unlocked
multiplier of the Mobile Barton.
The mobile barton also uses lower voltage (thus less power
and heat, "often" more overclockable) but it needs a
motherboard that can manually adjust the muliplier even to
run at it's stock speed most often. If your motherboard
supports this then a Mobile Barton may be the best choice
even if you don't overclock it.
Bartons have a larger core surface area too, are easier to
cool.
As for stats, there is only one of very great importance.
The Athlon is statistically most likely to be