Actual owner here: I own both the rMB and the rMBP. At first, I was sure I would keep the rMB. I brought it to work today and was pretty frustrated to be honest.
The UI lag was worse (not a ton, but enough) than on my rMBP. I tried everything to make it equal (yes there is still some on the rMBP with yosemite). The actual chin? on the rMB made typing awkward for me. On the pro I rest my wrist on the chin surrounding the trackpad. With the rMB it is too small to do this. I had to find somewhere to place my wrist. This may not be a problem for someone on the go, but sitting at a desk and typing on it felt weird.
The screen was also just small enough that it was bothersome. I used it as a laptop at work today vs. actually docking up like I normally do. I wanted to get a good feel for the screen size. On paper 12" and 13.3" does not sound like a lot, but it really is.
SSD speeds are widely inconsistent. It may not matter in real life performance, but it messes with my mind (that is what matters to me). I fear that if I had problems with the UI on the current OS, would there be a problem in the future?
All in all I have decided to keep my rMBP. I do actually like the feel of the keyboard better than the rMB. Both Keyboards are good, I just like to "feel" the keys if that makes sense.
I was all about the one port idea so I would not have an octopus rMBP at work but, it is both faster (noticeable IMO) and the screen size different is enough for me to want the pro instead. Oh, and the trackpad on the rMB does feel different than the 2015 rMBP. It is more sticky. My finger didn't travel as well across it.