I think for your purposes 1DX may not provide much more than 5D3 (and cost ~$3000 more).
I own both bodies, and hands down 1DX is a better camera all things factored in... although the 5D3 is no slouch; but whether the 1DX is a necessity for you in a studio setting is rather questionable.
You will find the 1DX sensor more forgiving, you can push/pull subpar exposures more than any of Canon's other contemporary sensors. 1DX trumps the rest of the line up in lowlight, sports, and wild life photography...you don't appear to fit any of those categories in first blush. In which case, 5D3 will offer you ~3 MP more without too many compromises and at significant financial savings.
Though exceptions always apply, and not mutually exclusive, here is a rather simple test to apply ...
If your actual lens usage on a day-to-day basis is overwhelmingly with 85L II, 50L, 35L, 24L, 14L, or fast standard zooms in this range then 5D3 may be an adequate fit, perhaps even a marginally better fit given the added ~3 MP's.
If you mostly use the 100+mm fast primes and super teles (in real use, not I have a 70-200mm f/2.8 II that everyone and their aunt has, and therefore I am a sports photographer logic
) then you will love the 1DX.
Just my 2 cents.