Being you have a rMBP you will be extremely disappointed in the 27. The 23, you may be able to live with however at that point you might as well just save your money and use the macbook screen.
I think this is good advice. I understand the pixel density issues that would occur on a 27" display with only 1920x1080 resolution --- especially when I'm used to starting at the gorgeousness of the 13" Retina MBP display.
I don't know, perhaps I'll wait Apple out even longer to see how their display situation shakes out. Another option would be to sell the rMBP and get a 27" iMac and an iPad Air. Would be pricey even with the sale of the rMBP, but would solve my problem.