Personally, how well I treat a girl depends on her sense of self-value.
If she already feels like she's a queen, I tend not to reinforce that.
The only notable difference is that I'll probably be a lot less complimentary towards the prettier girl. Why tell her what she probably hears all the time anyway? She knows she's pretty. I'd likely be more speculative over whether she has a winning personality to match.
They look good to me if I am taking them on dates. Some better than others? Sure. They are all good enough looking for me though. Looks are not the only consideration. Ie who gets more time? Unless the guy is superficial, he will spend more time with one woman over another for the same exact reasons he spends his time with one male friend over another male friend instead of looks.
If I rated the people I have been with by looks alone, the ugliest was the only one that I actually married.
Honestly, I don't think it matters in the long term, only the short term. Initially, if a guy is attracted to a really hot girl - he will invest a lot more into trying to get her. He will dress up nicer, or take her to a fancier restaurant.
But in the long term, I think his personality wins out. No matter who he's with, he'll treat her the same. If he's a douche and treats her bad he always will no matter her appearance and if he treats women well, it shouldn't matter her appearance