Yeah, and I'm telling you very clearly you're going to be disappointed with both products. You don't buy a Mac for gaming.... any mac even if you have copious amounts of cash. Christ, if you have that much money to waste, you're better off getting a pimped out 2012 Mac Pro and slapping in a GTX 770 or better for gaming and it wouldn't cost you anywhere near as much as that unupgradeable trash can OR the iMac with its lousy soldered GPU and CPU.
Just look at the performance of DUAL D700s when they go up against a single GTX 780 Ti;
Those D300s are going to get completely wrecked. So again, no, neither system is suitable for your needs.
To make matters worse, not all games play well with SLI/Crossfire;
http://www.anandtech.com/show/7603/mac-pro-review-late-2013/10
So in some cases, you're going to see no benefit at all from a second FirePro GPU.
So, logically and rationally speaking, you're wasting your money if your buying a Mac out of vanity. If you really need to have that 'pretty case' then get a 2012 Mac Pro and put some proper GPUs in there and you'll also have the benefit of changing them later or putting in additional ones in the future.