Hey. First and foremost, if you can afford to go with radeon 580, get that one. UE is as GPU reliant as it gets.
Second of all, if you can wait, I suggest waiting for updated Surface Studio. Keeping in mind that this is a win10 machine.
Why? I’ve made semi-scientific tests and found out that current state of rendering performance is 40% lower on my iMac in MacOSX vs win10 (around 21fps vs 30fps in some scenes, around 30fps vs 50fps in other scenes, you get the point). Same machine, different OS, wildly different results.
I particularly asked on the forums if any significant (or at least fair) improvements are to be expected with Metal2 shaders and yada yada, but Unreal Dev responded that I should not expect improvements on the machine I have in MacOs (a polite version of admitting the macos inferiority for realtime rendering).
If you are very bound to Apple ecosystem (like, you literally work in sketch3 and make money with it and it does not run in any other os, or you make movie soundtracks in Logic Pro X) than get iMac with the most powerful GPU. Those $200 that you will ‘overpay’ will go a long way in beefing up your speed and maximum visual fidelity.
If you are not bound to OSX and mostly use cross-platform solutions (like me), either wait for an updated Surface Studio – if you prefer the AIO solutions (with comparable specs it will run UE4 faster since it is a win10 solution) or just throw those $2500 at a ‘classic’ desktop and get a machine so powerful it will make you cry (with tears of joy).
PS – I find it mildly amusing that many things run better on win10 on my iMac (including Substance stuff). I also happen to own the last iMac that had nVidia GPU, because AMD solution does not offer CUDA cores for GPU rendering engines like Octane / Redshift, and AMD ProRender page has not been updated since Dec 2016, which is not that inspiring if you ask me. So, think about it. I still love my iMac, don’t get me wrong, but latest developments and decisions make me rise my eyebrows.