Is milestone or experience leveling better!?

Their are two ways of leveling up in Dungeons and Dragons. One of them is milestone leveling, where if you hit certain points in the campaign you level up. The other one is experience leveling, where each monster has as set amount of experience you gain that you get when beating the monster! Which one is better?----------Music provided by Kevin MacLeod (incompetech.com)Licensed under Creative Commons: By Attribution 4.0 License http://creativecommons.org/licenses/by/4.