Sempron 2600+ without a question. Once you get a K8 there is no going back.
It will kill the 2500+, and since it is 90nm SOI it will overclock quite nicely.
Plus, you can upgrade the Sempron to a socket 754 Athlon 64 later on that are still very fast; the platform still has some life in it. Socket A is EOL in march.
EDIT: I still stand by my answer but I realized that he is talking about a socket A sempron. In that case, the barton 2500+ is better for sure.