I was sure it would be a Nikon for you, Rod, and the D750 looks like an excellent camera. Once you have committed to a particular premier brand of (D)SLR and have amassed a collection of lenses for that brand, it is virtually impossible to change. I am a Canon devotee as you know, and I am unlikely to change my allegiance, having acquired a lot of Canon lenses over the years (not because I think Canon is any better than Nikon).
I am not sure how far the backwards compatibility of the Nikon goes, but the current Canons will work with all Canon EF lenses going back to 1987, not bad in my estimation. Unfortunately, many of the older models of independent lenses (Sigma, Tamron, Tokina etc) cannot always be relied on to be compatible with the newer Canon bodies. Hopefully Nikon may be better in that respect.
The good low-light performance of full-frame cameras is a well-known feature, presumably because the area of the photo-sensors is so much bigger. More photons hitting each pixel site must be a good thing. I look forward to seeing the results from your new toy, but I am sure you will still use your crop-sensor Nikon as the mood takes you.