Personally I'm selling my D3s to add a 750 to my 800E. The 800E is used far more often than the D3s now as it is - the 12MP of the D3s simply won't cut it for cropping purposes and it is very rare I actually need 9/10 fps.
The 750 covers most bases ; 6.5fps (vs 10 of the 7DII), 51 focus points (65 of the 7DII but the focus system of the 750 is I believe that of the D4 so no problems there !), great VF (much larger than 7DII), Hi-res screen (20% higher than 7DII - and I like the flip-out screen too, very useful on a tripod), downside is the 1/4000 shutter speed - but that is only 1 stop so can be recovered by lowering the ISO one stop, battery life is 1230 shots (vs, 670 of the 7DII) and the shutter is whisper-quiet - a real plus in bird photography !
The 750's high ISO and DR is also demonstrably proven in numerous reviews, 7DII we will have to wait and see if it delivers in the field.
Both are weather-sealed, both have continuous focus video and both have 2 storage slots. Where I live the price difference is negligible, the 750 is ca. US$150 more expensive. I guess that's similar elsewhere too.
The 750 also has a built-in flash, is lighter (750 vs 910g) and has in built-in focus motor (very important for use with the whole system's lenses) if any of those are important to you. If I was already using Nikon then I'd take the 750 over the 7DII because I don't see the Canon as offering enough to consider a change.
Personally I would not change my whole system anyway for one camera that will be matched / obsolete in a year or two, I've too much invested in my Nikon lenses. If you are already using Canon it makes sense to go for the 7DII because they don't currently have a good prosumer birding camera at the moment though my birding buddy is happy with his 6D.