The only real difference is what type they turn into and what sort of moves they learn. Their behavior doesn't change much.

[Or it shouldn't, anyway. ...even if he may end up throwing fireballs at people he doesn't like if he chooses to evolve into a Flareon.]

No. My partner and I moved on towards Azalea about ten days ago. It was too last minute to tell anyone. [Which is why there was no warning. She's not dodging Page so much as helping her brother dodge a gym leader with a crush.]