I have two crop and two full frame bodies, and for me it usually comes down to wanting the wider angle coverage I get from a given focal length lens. Walking around Hoover dam with a FF and my 24-105mm resulted in an excellent zoom range. A 1.6 crop factor would not have worked as well. Shooting a dance floor with the 6D and 70-200mm f/2.8 II wide open was again the correct combination. In that case the zoom would have been way too long with crop body. Often shooting wildlife the crop is a better choice, due to the extra reach, though I have done better with birds in flight with the 6D and 400mm f/5.6 than with the 60D, for some reason. Having started with crop and now having both, I would say I grab the full frame 80% of the time. On a trip in the car I usually try to keep three of the bodies with a lens attached so I am not fumbling with mounting a lens. Unless you simply have no choice from a monetary standpoint, keep your existing body and add a frill frame.