Number of cores, individual core performance, cache size, or clock speed? I’m sure it’s a combination of things, but what aspects of a CPU affect XPM mining ability the most?
It´s in fact the combination. However, more cores in mining works better then for other applications. Beside the clockspeed the architecture of the CPU is important. I would look at benchmarks, to decide which CPU does have the best value. Hyperthreading is not only worthless but bad for the performance. This is because it takes to much cache to run two threads per physical core.
I read a few times that AMD CPUs do better then Intel for primecoin mining (but didn’t test this myself since I only have Intels)