For me I'll go for 735, better front facing camera, batter rear camera (Ziess optics + HAAC mic), looks more beautiful.
735 lacks the following:
Glance screen, Lumia camera 5.0, update 2(it's not a big deal).
Why I won't get the 640?
I love selfies, Zeiss optics is a big deal for me (I'm currently using 720).
For Lumia camera 5.0, this will get it with Windows 10

(as we know, maybe sooner)
Tell me if you want to know more!