A word of warning, Macs are terrible for 3D work. The GPUs are severely under powered, but that's just the start of your problems. You soon run into Apple's truly awful OpenGL support. It's slow, staggeringly slow when compared to Windows (running on the same hardware), it's buggy, crashy, and doesn't support vital functions such as tessellation.
OpenGL has of course now been superseded by Vulkan, which Apple refuse to support. With no Direct 3D on tap either (for obvious reasons) they leaves devs with nowhere to go. Apple have been pushing their own locked down proprietary API called Metal, but as it's Apple only and not as low level as Vulkan or DX12 it has absolutely zero take up from pro app developers (or even OS X games). It's 100% dead.
Even 2D stuff struggles on the Mac to be honest, and I put that down to the weak GPUs again. The 5K screen on an iMac is staggeringly good, but if the GPU can't drive it quickly enough, it's a big problem. Photoshop bogs down quickly with heavily layered files and/or larger documents on a 5K iMac. 5K screens are rare on PCs, but I've seen 4K screens running similar Photoshop files and they're so much quicker under Windows.
Even pure CPU applications like zbrush, which should theoretically be as quick on OS X as on Windows are slower. No idea why.
All of this is regarding high end work. If you're just dabbling with simple stuff you'll mostly be okay, but always remember that Windows is fundamentally a much, much faster OS when it comes to graphics.
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Get what make you happy. If you want a pro level Mac for the hard work then get a Mac pro, it will last much longer and do a much better job than a iMac.
The current Mac Pro is comically out dated.
I honestly don't know where Apple are planning to go with the Pro. The GPUs in it are ancient and severely limited by size. There's no way they could fit a couple of 1080s in there. Then there's the Vulkan/OpenGL issue, which is a real killer for the pro market.
I suspect Apple's fleeting re-entry into the pro sector was a one off, and the Pro will be left to languish as the 27" Apple display did for years without an update.