here is a benchmark website, you can click on the 64-bit multi-core tab to see the mac's you are looking at. Mac Benchmarks - Geekbench Browser
you may find your current macbook pro on there and the new one, to compare those too. if you can decide how much performance you want vs the cost, it may help you decide which way to go.
a large part of the cost of the imac 5k is the 5k display. the non 5k imac with an i7 is a good value, but only has 16gb of memory as an option. the mac pro is expensive because its workstation grade hardware.
for memory in the new mac pro, many order the 16gb version and replace it with a third party 64gb kit to save alot of money. having multiple programs open with large resolution projects will use alot of memory. the 32gb on the imac may work fine, but may also test the limit depending on whats being done. the 64gb on the mac pro would have alot more breathing room, but with more cores, will also use more memory to feed those cores.
the dual gpu's in the mac pro would offer alot more performance, but only for programs and plugins that can use multiple gpu's. premiere and speedgrade can, but i don't believe other adobe programs can. you would have to check all the plugins and software you plan on using. if they don't, a single firepro d700 would still be faster than the imac r9 m295x.
Thanks for your reply Ronin, those benchmarks were quite helpful in my considerations. One thing that really surprised me was the performance of the imac when using a single core, it actually scored higher than the 6 core mac pro in that regard. It was also surprisingly close when using multi-core. And seeing as how Ae is no longer taking advantage of multiprocessing, I don't know that I would get any use out of those extra cores. features not available in After Effects CC 2015 (13.5) | After Effects region of interest
So, I think the iMac is probably the best bet for my dollars, and I can translate the saved cash into a new laptop sooner than planned, so I get the best of both worlds.
For other people interested that might be reading this, I also found this really helpful video : 5K Retina iMac VS 6 Core New Mac Pro ~ 4K Video Editing Comparison - Final Cut / Premiere Pro - YouTube It deals more with performance with video editing, but the specs and benchmarks are still mostly applicable, especially to rendering, since Pr uses media encoder, and that is a big part of my Ae workflow.
AE 64-bit was always plagued with non multi-threading code, but they removed the work around (multi-processing) in order to move forward and fix AE. last i saw the folks in the AE forum were recommending using cc2014 to have a functional version. once adobe is finished fixing AE, it should finally use all cores and memory requirements may drop some. when that will be done, i dont know. so AE users may be stuck with cc2014 for a while.
the single core benchmark is what it should be, a single 4ghz core from the 4 core imac should be faster than a single 3.6ghz or lower core. intel has been adding more cores vs while dropping clock speed in its larger core count cpu's. single core benchmarks shouldn't be of any concern in today's computers, but unfortunately there are software companies like adobe which don't code properly, leading to multi-threading issues with their software. when this happens 4 core systems outperform 6+ cores.
i watched the video, looks like there were some multi-threading problems in fcpx for some of those tests. any benchmark or program where the imac is faster, is a sign of software multi-threading issues. the exception was the h264 tests, in which fcpx was able to use intel's quick sync. thats one feature adobe should have implemented years ago, but sill hasn't.