I'm waiting this one out to see what the SP4 has when Win 10 launches. I much prefer OS X, but I think the advantage is much closer than what it has been in the past.
The key thing holding me back right now is the rMB is the better looking, hands down, but with the SP4 (or SP3) I can combine a laptop with my 128GB iPad Mini. I would much rather have one device that I can play the 4 games I play and do some excel and photoshop work on a flight than have to carry two devices.
My favorite machine is my 15" rMBP, but for my "ultra portable," I really care more about the features than the OS... They both "work."