Your answers surprised me. Didn’t have the feel from your first post that you would be into post and therefore be concerned about noise levels. Combining that with your sensible desire to buy into a system that you can build on, I’m reluctantly going to vote in favour of FF.
There are so many practical upsides to crop sensor systems, that I was reluctant to move to FF myself. Just one very significant advantage to APS-C and 4/3 is getting landscape DOF in the f/7 to f/8 aperture range, instead of f/11+ for FF.
But in counterpoint, Canon, Nikon, and Sony are short-shrifting APS-C and putting their lens dev energy into FF. The good glass from these companies is increasingly to be had at the FF end. Apparently, even Pentax/Ricoh is now no longer denying rumours that they're working on a FF model. So the future product landscape looks to be increasingly polarized toward FF for top-end equipment, with anything smaller being relegated to budget/beginner (of course, with Fuji, Olympus, and Panasonic as hold-outs).
I upgraded from the Nikon D7000 (16 MP APS-C Exmor) to the A7 (24 MP FF Exmor) a year ago. I was already very pleased with the D7000’s IQ, and was not prepared for how large an increase in IQ that change bought me. Now I literally have to shoot north of 800 ISO to see even a slight noise granularity at 100% mag. In theory, you would only ever have to shoot a 24 MP APS-C — as in the D7200 or A6000 — opened up by a single stop more than the same shot on a FF to get the same results. But I’m not prepared to believe that at the gut level after this past year’s shooting and editing.
More than once I’ve bought home a card with shots underexposed by a stop or more or overexposed by a stop or more. Either way, the files would have been compromised or ruined had I been shooting with any previous camera I’ve owned. With the A7 it’s just a matter of tweaking a few sliders, then I’m scratching my head trying to find where I’ve lost IQ. A Sony evangelist coined the phrase "full frame forgiveness" in reference to this. Amen to that.
I have zero experience with Canon SLRs, so can’t speak to whether you’d be quite as amazed by moving to a recent Canon FF as you would a Nikon or Sony one. Canon’s FF bodies are anything but petite — a serious concern for your backpack. But there’s a lot of evidence that Canon is still the system to beat in every regard except, arguably, certain aspects of IQ. A close correspondent in the US has been buying/trying/selling a lot of different APS-C cameras from Canon, Nikon, Fuji, and Sony this past year. He loves the Exmor IQ but hasn’t found anything from Nikon or Sony to dethrone Canon for usability, affordable lens excellence, and customer support.
Quite the quandary, so no simple buy-this from me!