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The Nikon D800/800E cameras are massive and I would suspect from handling the Canon cameras too large for my hands. Additionally, going the Nikon route not only means Leitaxing the lenses but it would surely be very inconvenient to go back to shoot film on my R4SP, R7, or R8.
Rich, I've got smallish hands and find the grip on the D800 and D600 to be too bloody small to be comfortable. Give them a whirl. I'm actually looking at the 5D's in part because of the larger grip (I may not like big bodies, but if I'm stuck with one, I want a grip that actually fills my hand), as a compatriot to my NEX-7.
Your R's in this case can be replaced by a FM2n/FE2/FM3a and a F100 or F6 for film shooting, and you'll gain capability, particularly with the FM3a/F6 combo.