I'd go with the p3 ! You may have a full rack now, but you'll thank yourself when you fill it up! The p3 is a better server chip anyway. If you could tell us what each motherboard is, we could give a better evaluation!
While the P4 does have active thermal throttling and the P3 did shut itself down, AMD now has features which do the same as well. But this catch is it is not on the CPU like Intel's thermal solution, but rather a function of the motherboard.
This again brings up the issue of what motherboard is being used.
I've heard too many horror stories about Athlon chips in web server roles. I'm living one right now with one client - we will be swapping out the CPU / memory / motherboard in the next few weeks because we're living one of those horror stories. I have an Athlon, motherboard, and memory gathering dust in my basement because of my own hooro story.
Performance ratings of the MP were not too great, I've read about 3 or 4 reviews of the MP as I was curious and all said that its Intel counterpart was a better component. I'd also get the P3 for space saving as already mentioned.