AMD Athlon 300+ has 512mb cache but runs at 2 ghz and the Sempron has has 256mb cache and runs at 2 ghz.
It depends, the Athlon 64 is more for gaming, while the AMD Sempron 64 is more for multi-tasking and used for simple programing. Both of them are fine great (cheap) prossecers and I recommend them.
Basically Athlon and Sempron is the same Difference as Pentium and Celeron Someone who wants to do a lot of gaming would have either the Atlon or the Pentium an office user would have the Sempron or Celeron. Really it comes down to what you would be using the CPU for. technically the Athlon is better but if your not using it to its potential then its no better than the sempron