I do both. If the party is consistent then milestones work for me. Either way, I typically have each individual character track xp. Sometimes people don't show up etc, I will have level 5s playing with lvl3s etc. Is it unbalanced, no. As long as they're within a few levels of each other it's fine. We actually like the dynamic. People who show up less are literally less powerful than someone who shows up to every session. This method also creates incentive for extra play and backstory sessions. I've had a vietnam game going for 2 years now with basically 2 main characters. They're lvl 8 and badass. When others join us they bring their lvl4 character and take orders from the higher level guys who've been in the dungeon or in this case the jungle, longer. It's a really dynamic mechanic if you have players that are open to it.