I think both techniques are being a bit pigeonholed in this thread. They are both actually very flexible.

People conflate Milestone with story beats, but it doesn't have to be. You can level up every 10 encounters, you can level up every 4 sessions, you can level up every time the party loot hits a predetermined value, you can level up every time a PC dies. Anything not based on counting experience points can fall under milestone advancement.

Likewise, XP can primarily be given out for story beats if the DM wants to do that. It's assumed that xp will be rewarded by killing enemies since they have explicit values listed, but it doesn't have to be. You can do XP and have the players just as much in the dark about how much XP they're going to get and when they're going to get it.

The main difference between the two systems is that XP is tracked so that players can see their progress to the next level. This system tends to be a bit more flexible because you can give out XP for literally anything - good roleplay, innovative ideas, ordering pizza for the table, whatever. But as such it also requires more work for the DM and pushes advancement to the front of everyone's minds because they are constantly tracking it.

Since "modern" D&D tends to value the story a bit more, Milestone allows more focus on this as advancement isn't really addressed until it's time to level up. But I think it's popularity is mainly due to the way it removes the mental load for the DM.

The good news is that both systems can work well. Find the one you prefer and tune it how you like. Most of the cons of either type discussed here can be mitigated with flexible thinking.

Quote from scatterbraind >>

I think both techniques are being a bit pigeonholed in this thread. They are both actually very flexible.

People conflate Milestone with story beats, but it doesn't have to be. You can level up every 10 encounters, you can level up every 4 sessions, you can level up every time the party loot hits a predetermined value, you can level up every time a PC dies. Anything not based on counting experience points can fall under milestone advancement.

Likewise, XP can primarily be given out for story beats if the DM wants to do that. It's assumed that xp will be rewarded by killing enemies since they have explicit values listed, but it doesn't have to be. You can do XP and have the players just as much in the dark about how much XP they're going to get and when they're going to get it.

The main difference between the two systems is that XP is tracked so that players can see their progress to the next level. This system tends to be a bit more flexible because you can give out XP for literally anything - good roleplay, innovative ideas, ordering pizza for the table, whatever. But as such it also requires more work for the DM and pushes advancement to the front of everyone's minds because they are constantly tracking it.

Since "modern" D&D tends to value the story a bit more, Milestone allows more focus on this as advancement isn't really addressed until it's time to level up. But I think it's popularity is mainly due to the way it removes the mental load for the DM.

The good news is that both systems can work well. Find the one you prefer and tune it how you like. Most of the cons of either type discussed here can be mitigated with flexible thinking.

 
This.

Seriously. If I ever finally figure a way to create an encounter system that doesn't rely on XP, I will switch over to my system for milestones, which is totally not a "RAW" or even a RAI" basis.

I wanted a way to separately reward those funny, creative, joyful moments, a way to reward and encourage the foolhardy, darn the torpedoes heroic actions with cinematic (descriptive) value, and a way to reward story progression, participation, and provide for level advancement, and have them all be able to interact with each other.

The parts were there already. I just hooked them up, and now I have what are basically 3 "Destiny" values of Inspiration, Heroism, and Milestones. Those are what we value -- but I could use milestones for all of that just as easily (I'd just lose the flexibility of all three can be used to change certain dice outcomes to a degree).

Milestones really are still XP. Just a different way of looking at the concept of gaining experience.