^^
I am actually in the same dilemma but I pretty much made up my mind. I have both the 2014 macbook pro retina 2.8ghz 16gb ram 1tb ssd and I have the late 2013 iMac 27" with 3.5ghz i7 32gb ram 4gb video card 3tb fusion drive. I decided to keep my macbook pro because it is closely fast and keeps up with my iMac. I definitely loved the option of going mobile so to supplement the absence of the 27" iMac screen, I went and bought a thunderbolt display 27". Haven't really noticed any lag using the macbook connected to the TBD.