This was something I faced a while back... so let me explain my thought process so you at least have another perspective.

Back in the days of building the Silver Hammer, I was figuring whether to go many GPU, or Big-and-Few GPU, very similar to what you're thinking. I had a computer at work that had two 5870s in it in adjacent slots, and dealt with a lot of heat issues. I had read enough on the boards to know heat was going to be one of my big issues. Bryan was also studly enough to loudly proclaim the HAF-932 to be awesome.

So HAF-932 would get me enough airflow to cool big cards, even with OC'ing. I got a Mobo with an extra slot between the PCIe slots to help with cooling. With the extra space, things would be cool. Operating under the KISS principle, 2 cards is easier to manage than 3, so I went with two. With limited PCIe slots, I went big, rather than bulk. The rest of my hardware could handle two big cards. Big may not have been the most economical choice (I actually refuse to do the calculations, because I don't want to know the answer), but it got me more credits/slot than anything else. Hindsight being 20/20, I don't regret the decision.

That was my train of thought. Also notice that JPM went big with the Maxwell Spanker...