I'm facing a similar dilemma. I'm running a 2009 Core i7 27" iMac and a 2011 15" MacBook Pro that has a Crucial 512gb SSD. I want to replace both with either a 13" or 15" Retina MacBook Pro plus a 27" display.
The way I see it, to gain the extra portability of the 13", I would be stuck with dual-core rather than quad core, and Iris 5100 vs Iris 5200 for graphics. I am waiting to see benchmarks for these new systems to get an idea of performance. Also, I did some research last night on how to limit Lightroom or PS to two cores, so I could get an idea of how it runs on my current MacBookPro on two cores and integrated graphics (using gfxCardStatus).
How did the test run on 2 cores?
Oh, sorry. I just found the solution late at night and didn't run any tests yet. Here's how to do it: