The best Mac Pro For After Effect : 2014 Mac Pro : 12core+2 And 6gig (total 12gig) GPU + 64 Ram OR 2012 Mid Mac Pro : 12core+2 Nvidia Quadro K5000 4gig (total 8gig) GPU + 64 Ram
Newer is better. More ram is better, GPU doesn't mean much in AE but it means quite a bit in Premiere Pro.
Right now the best system for AE is the X99 since it has DDR4. DDR4 has far greater bandwidth than DDR3 which impacts AE for most workflows.