which is better
It’s a closer call than your previous comparison - they may both win some benchmarks - but I’d still pick the Sempron (are you looking at a Socket 754 or AM2 version?). Just a feeling, I haven’t checked anything. If you choose a Socket 775 mobo that can handle Core 2 Duo (or even Quad Core), you might want to go for the Celeron now anyway and upgrade later. If the Celeron is a Socket 478, forget about it.