Huh, there's more endings than I thought. Took me 2 sittings to beat the game, still pretty early for today.
So the majority of dialogue choices you make have no bearing on the ending of the game. I told Ryan I don't love him everytime, but she still did. What does matter is who you save or who you let die. Or you can just kill everyone by not making it to the end in the last 5 minutes of gameplay.
There are some story-changing decisions, though:
When Aidan first possesses your minder to take you to a club, if you sit in the front of the car you get caught on the way out and that's it, but if you hide in the boot, you actually escape, go to said club and (apparently) get drunk and almost get raped, preventing you from having any romance stuff later on. That's what I heard, anyway; the playthrough I saw just mentioned that it was possible.
Sitting in the front can still get you to the bar. You just have to ram the guard station gates. That's how my playthrough went.