You should definitely get an Athlon over a Sempron any day. Barton or Thoroughbred core, both will be way ahead of the Sempron. I have no idea how much the AthlonXPs go for these days (if you can even still get them :D), but it's probably worth it over the Sempron :o
To put a bit of balance on it, a Sempron 2600+ will be about as fast as a AthlonXp 2000+ or something along those lines. Not sure on an exact figure, and it'll vary depending on what you're doing, but you should get an Athlon (Y)