Full Frame.
Image Quality: Yes a lot of APSC cameras have great image quality, but I'll always prefer more sensor real estate for my pixels to inhabit.
Low Light: FF seems to me to work better in low light.
Focal Lengths: My 85mm is 85 on my film SLRs and 85 on my FF. Also not losing a chunk off the wider angle due to crop factor.
Feel: It's an intangible, and one I'm almost hesitant to add, because I do like working with several smaller sensor cameras too, but there's feel to FF, that I don't get elsewhere. There's a chance that's the placebo effect, but I don't think so.
and lastly
DOF: I have more options for shallow DOF should I need to, it's not something I'm trying to do all the time at all, but its nice to have that ability.
FF bodies aren't THAT big.
I appreciate that the Leica and Sony models are appreciably smaller than the Canons and Nikons.
I don't think the Leica feature set is comparable to the FF Canons and Nikons, it's for different use. I've yet to see much in terms of output from the Sony, that's no judgement on it, I just haven't seen enough from it yet, it looks very interesting.
I have a Canon Eos3, which is a chunky 35mm slr, but that's because it's a lot of camera. The 5d mk3 is the same size.
There's 15 years between them, they're the same size, they feel comparable in the hand, and the 5d manages to cram in all the video stuff as well. It's simply not true that SLRs are these ever inflating beasts.