The biggest difference in the notch, obviously.
Second biggest one is in the rear camera. 6 Pro has a larger aperture (f/1.9 vs. f/2.2), larger pixel (1.4µm vs 1.25µm) and lastly it has dual-pixel technology which The 5 Pro lacks. Selfie camera also has a larger aperture (f/2.0 vs. f/2.2) while the secondary telephoto lens seems to be the same.
So, The 6 Pro should be able to capture a LOT better images than its predecessor in dark environments; at least in theory.