Don - Are you finding the shutter bounce to be a problem? I am following the same logic set as you and have yet to make up my mind. Came close but talk of shutter bounce at anything under 125th of a second and the variable performance of each Metabones adapter has made me wait. What has been your experience?
I too read the reports of shutter bounce but so far I have not noticed it to be an issue for me. I've also only had it for a couple of weeks and I'm still running it through a range of shooting scenarios and conditions to see how it holds up.
I ordered the wrong Metabones adapter (Speed Booster) at first and had to swap that out for the proper one (III) and now it's fine. So far I've tested 14mm f/2.8L II USM, 24-70mm f/2.8L II USM, 16-35mm f/2.8L II USM, 70-200 f/2.8L II USM (also with a 2x extender III) as well as the 8-15mm f/4 Fisheye and all work with full functionality (albeit slowly) on the metabones. The only Canon lens I own that did not work is my really old 50mm f/1.4 which I knew going in was not supported due to the age of the architecture.
I'm still working through the use cases to confirm which camera body is best suited for which of the types of shooting I do. Right now, any type of landscape shooting I do either handheld or tripod is likely to be on the A7R and anything handheld that needs fast performance will use the 5D3.
For example this past weekend I shot an airshow and have another one coming up on Sunday. I won't even bring the A7R with me for that sort of condition since it's way too slow all the way around but the 5D3 can handle those conditions just fine. However I've got a sunset/dusk cityscape shoot planned for this weekend and A7R will be the primary tool for that shoot since It'll be on the tripod the whole time and I want the greater DR it's sensor offers.
For me I think it's going to be about picking the right body for the proper use case and having the metabones adapter gives me the option of using my large investment in L-glass either way. A friend of mine described it the other day as being like I had the option to 'swap backs' behind the lenses based on the shooting conditions.
lots more testing to do and lessons to learn but so far I'm quite happy with it.