Why are you getting rid of the Sigma ART? I believe it's sharper than the Nikon 35mm 1.4. I'm pretty sure Sigma can swap the mount for you for a small cost (from Canon to Nikon).
Keep in mind that the D750 has been around for several years now, I wouldn't be surprised if there's a replacement on the horizon.
I shot a D750 for a few years (I bought it shortly after release) and am moving back to Canon. When I throw all the specs and features out the window and look at the images I loved best, I had more from the Canon camp than anywhere else. Data points are really good at telling us how much DR a camera has, but it can't measure the intangible parts of an image such as how it's rendered, the 3d effect, etc.
The 135 is truly magical. I would consider renting the D750 for a weekend before taking the plunge.