Well the AXP has a faster core (but not by much) and more L2 cache (512K vs 256K) so the AXP will be the faster processor (at stock speed) but as for your question ?Which would be better?? it all depends.
Better for what? performance, price, overclocking?
Peformance stock speed = AXP
Price = Sempron
Overclocking = possibily Semperon since it is has less cache and newer production, but I cant say for a fact.
Performance overclocked = depends on the level of overclocking you are able to attain with each, and how much effect the L2 cache has on the programs you run.