It depends on the workload and what "significant" means to you, but a Mac Pro is generally going to be a better performer for heavy AE usage.
AE only currently supports CUDA (i.e. nvidia GPUs) for GPU rendering, so that's out. AE will make use of lots of cores, so multi-threaded performance is the key.
Checking out the GeekBench 64-bit Multicore benchmarks is probably the easiest gauge on paper, but keep in mind that an iMac has a lower thermal threshold, so it will likely start to throttle (and/or a lot of fan noise) over an extended continuous render, while the MP can go for days at full throttle without slowing down.