If you use molecular dynamics packages or you can use cuda programming, I suggest to pay for better GPU instead of CPU. GPUs are very faster then CPUs i parallel programing.
GPU is very faster CPU, but not all the softerware or program can run with it. For molecular dynamics simulation, some potentials can not run with it.if you can sure your softerware or program can run with GPU, it really the best choice. if not, choose CPU. Intel Xeon run more stablity than Core,but basic frequency is lower. if you need strong computing power, HPC may be more suitable