This article spells out the evolution of Quick Sync:
https://en.wikipedia.org/wiki/Intel_Quick_Sync_Video
The base iMac isn't all that powerful. It's clocked at 1.6GHz while the base MBP is clocked at 2.7GHz. That is a huge difference. The i7 processor is much more powerful overall being a "virtual 8-core" machine (using "Hyper-Threading" which is very different than "Turbo Boost"). [Just sayin'... it's available on both machines.]
Sometimes these nit-picking issues make a difference.
According to the linked article, Quick Sync starting with the "Haswell" chips are optimized for speed. Even though Macs have been using Core i5 and i7 Intel chips for some time now, each generation has been markedly "more efficient" (faster — better than can be accounted for with clock speeds.)
If you're buying for speed and you don't need a laptop, then "invest" in the technology that will deliver that speed (higher end iMac.) I also recommend at least the Fusion drive, plenty of RAM and the best GPU you can afford (greatest amount of vRAM available.) [The GPU choice alone will narrow down which Mac will have to buy most likely.] The last is NOT least! It will determine how long your Mac will be "current" down the road. FCPX and Motion are exceptionally vRAM hungry. Laptop bus speeds are notoriously slower than their desktop "equivalents" (power differences, disk drives, etc.) and the true speed of any machine is the bus!
In the case of Mac vs PC, it's the OS differences. Windows cannot divorce itself from the wide variety of hardware options it has to deal with. Back in 1995, I had a 16MHz SE/30 that could out-perform a 150MHz Windows 95 IBMPC easily. QuickDraw was an amazing core graphics library. Back in those days, it was burned into ROM; an advantage Windows machines have never had because the hardware and the OS were never from the same company. Similar comparisons can still be made today.
