If it were me, I'd stick with 5D3. 1DX and 5D3 got the same AF (almost). As for low light performance, 1DX is just a little bit better. I suggest getting 1DX only if more than 6fps performance is crucial to you. Your macbook is also important since it is so good to edit raw files using mac. If I were you, I'd like a good TS-E lens instead of the 35 though.
I agree with this advice....EXCEPT the bit about 35mm. if you shoot a lot of product and architecture then, yeah get a TS-E...if not... save a bundle an get a better lens in the Sigma 35mm f/1.4. ...there is $1000 for a nice trip to use it on!
5D Mark III, Canon 15mm f/2.8 Fisheye, Canon 17mm f/4L TS-E, Canon 16-35mm f/4L IS, 21mm f/2.8 Zeiss, Sigma 35mm f/1.4, 24-70mm f/2.8 II, 50mm f/1.4 Sigma Art, 85mm f/1.2L, 100mm f/2.8L Macro,70-200mm f/2.8L IS II...1.4x converter III, and some other stuff.....