Your best option is a custom Windows PC for the tasks specified. That way you'll have the ability to spend some of that money on a second Xeon CPU, which will be a greater help in AE 2014 (NOT 2015) than anything else you can buy.
And you'll get the added benefit of being able to use a CUDA card that does both CUDA and OpenCL for all the different kinds of 3D renderers that tend to support it. Despite Nvidia making a string of recent missteps, having both CUDA and OpenCL is far more beneficial than only having OpenCL via a (sometimes) cheaper AMD/ATI equivalent.
Mac Pros do not have the option of Nvidia cards, therefore no CUDA.
The new iMac uses a MOBILE version of the old generation of AMD/ATI cards, which is relatively gutless compared to even 2 year old desktop cards.
The new iMac's are also not fond of working flat out, under full load, for long periods of time, either.
Fortunately for you, Windows 10 came out today. Which is not nearly as painful as Windows 8. Get some water cooling with all your money, enjoy a silent room and the best performance you can have in a workstation.
Plus, all the best 3D software works on Windows, some of it doesn't work on Mac.
Think 3ds Max! I'm not even vaguely biased
The choice of OS is a very personal one but you can't beat Apple's customer service. If you are talking about the a system that will serve you well and holds it's resale value right now I'd probably go with the iMac over the Mac Pro. Adobe is constantly working on AE and it's other apps. Initially the MacPro was under performing with AE but that is improving a lot with each update. I know a lot of production houses that are using iMacs for a lot of pretty heavy projects. I don't know of any professional 3D software other than Autodesk's suites that doesn't run as well on a Mac as it does on a windows machine. One of my biggest clients, an international civil engineering company managing huge projects runs all of their Windows only software from Autodesk on Mac's booting to windows because the machines have shown the greatest reliability in the field because they are so well manufactured. It's all up to you.
It's true that you can get a little more horsepower with a custom built machine but you also have to figure in the cost of maintenance, trouble shooting, repairs, and all other expenses associated with running a computer for a business. I have owned many factory built Windows boxes and a bunch of Apple machines over the years and in the long run, as a business owner, I have had a better ROI on all of my Apple gear than any of my Windows boxes, even very expensive ones from major manufacturers. If you are just doing AE as a hobby and you are not keeping track of the time you spend then by all means build your own machine and save a buck or two.
Johnny Armstrong wrote:
I have read an article that states that after effects will not use the multi cores in a Mac Pro, is that true or not?
Not true in the spirit of the question. It has nothing to do with Mac Pros at all. AE performs on them the same way it would on other machines with multiple cores or multiple processors. PC, iMac, or Mac Pro - it's irrelevant. CC 2015, as dissidently points out, does not use multiprocessing rendering, while CC 2014 (and earlier) does. The reasons for this are extensive. You can read a bit about it here if you're interested. At home, I work in CC 2015 and render in CC 2014 since the project files are compatible between the two. Now, multiprocessing in any version of AE is a tricky affair. Some expressions or effects aren't compatible with it and you'll be back to rendering on one core for that composition. So...sometimes you could use all the cores and sometimes you couldn't. (This is probably a big part of the reason for AE's big changes mentioned in that link I gave earlier.)
Johnny Armstrong wrote:
i would like very very fast rendering and a dedicated graphics card.
What sort of heavy 3d work do you plan to do? Many 3d apps don't use the GPU at all. Take C4D for an example. You can get third-party renderers (like Octane) that do render on the GPU (and it looks like a great tool - I'm considering getting it), but native C4D barely touches the GPU.
Element 3d uses the GPU, but native AE doesn't (except for accelerating the ray-traced renderer, but that is an obsolete feature and you can't do it with AMD cards anyway). GPU (CUDA, OpenGL) features in After Effects
I also agree with dissidently about Windows vs. Mac. I used to love Apple's products, but I've been unimpressed with their professional offerings for the past several years - pretty much since they stopped exhibiting at NAB. They don't seem to be building things their professional users actually need. In the meantime, Windows has been getting better and better. Windows 7 is extremely stable and runs very well with all the pro software I use and, from what I hear of Windows 10, it runs like Windows 7, but uses less RAM while it's doing it! A lot of companies out there (Lenovo, HP, etc.) offer some excellent workstations that rival the Mac Pro's specs for significantly lower prices. Rick Gerard is correct that resale value of Apple products is better than Windows ones, but I think that's offset by the cheaper initial cost.