It's strange how our imaginations parse things, isn't it?
In the first adventure of my next campaign, I've already decided that everyone will gain one level when they escape the introductory dungeon. Whichever exit they choose (there are two), and how they reach it, will reward them with a level. Their choice will have a huge impact on which direction the story goes next, but the XP reward will be the same.
If I announce that to the group at the end of the adventure, you are suggesting that some players might feel like their choices didn't matter. "You have escaped the dungeon of Black Mountain! Congratulations, you all gain one level." I think I agree: that sounds really stiff and flat.
So if I were to instead award 300XP, the players might feel like their choices mattered a little more. Heck, I can use XP to make the milestone leveling all but invisible, if I'm clever about it. Maybe at the end of the night, I cherry-pick 3-4 memorable events from the adventure and give them enough XP to just barely pass 300. "...for successfully sneaking out of the torture chamber, 100XP. For surviving that goblin ambush and the dire rats, 150XP. And for, um, finding the secret door and disabling its trap, another 60XP. You all get 310 XP total."
Either way, the party goes up a level once they escape the dungeon, just as I had planned all along.
The really important parts will be stuff like...did they kill the boss, or let him escape? Did they take the exit that leads to the swamp, or the one that leads to the forest? Did they rescue the other prisoner? Did they find the sword? That's where the choices they make are going to shine (or haunt them). I'd prefer they focus on those rather than the points they get for killing Whatever, but I can see where assigning milestone levels might pull focus.
Thanks for the food-for-thought.