Think of it this way, there are Intel Pentiums, and Intel Celerons. With AMD, there are AMD Athlons, and AMD Semprons.
I don't think this response could be any further from the truth, though I do understand you have good intentions. Yes, the Sempron and the Celeron are budget classes of CPUs.
However, per passmark there's only a ~10 point advantage to the P4. I also know for a fact that on a P4-2.4, I can't run 30 bots on a CS server whereas my Sempron 3000+ can. Granted, the P4-2.4 is an older P4 and the 3000+ is a faster Sempron than the 2800+... but the point is the name of the CPU means almost nothing.
I would first of all check what else comes with the system, any bonus in RAM or anything like that will immediately tip the scales in favor of that system - the P4 isn't sufficiently better than the Sempron to warrant picking it for no good reason.
If everything else is equal, I would take the P4 personally. I'm a fan of AMD, but in the dedi market, especially back when those machines were the "hot thing" there were tons of cheap mobos/chipsets floating around for AMD systems... and provisioning Intel typically meant less problems. I'm not implying the AMD system will necessarily have a garbage chipset, just that the Intel would be a safer bet.
TL;DR: OP didn't give enough information to make an educated comparison.