I am getting the 750 for review in the next week or so, so maybe my opinion would change, but I typically prefer no screen.
Honestly I don't know if the bulky front cam of the 750 is worse than having a 720p rear cam. I feel like everyone is so concerned with video quality when 99% of the time all that really matters is seeing who hit who.
Voted for DR750 but only because it's the only one with Full HD 1080p Front and Rear. I didn't want the touchscreen and the unit is too big to be honest but having Full HD Front and Rear was more important to me. If there was a newer version of the DR650 that had Full HD on both, I'd go with that.
Having said that, my DR750 is my second BlackVue and will probably be my last! The units have great functionality but reliabilty (IMHO) is very POOR!