lf l ever plan on dming, should l use milestone or exp based leveling. can someone explain the pros and cons of each.

I use xp, but that's because I'm a numbers guy. It all depends on the style of play you choose. If your campaign is more structured to encourage heavy roleplaying and and a strong storyline, I would recommend a milestone style because I feel it would better allow for a dm to justify exactly when the players level up. When a character levels up in the middle of a fight because they killed just enough zombies to level up, it might kill the fantasy to pause and read through stats. It's all up to you and whatever feels best, and if you try one way and aren't liking it there aren't any rules against changing your mind and switching it up. It is your game after all, just have fun.

When a character levels up in the middle of a fight because they killed just enough zombies to level up,

Just an FYI, RAW that shouldn't happen because you award XP at the completion of an encounter, not on a kill-by-kill basis. So if you have an encounter of 6 zombies, you would award the XP for defeating those 6 zombies once they were all defeated.

It all depends on your personal style.  If you don't mind dealing with math, EXP based leveling is an excellent way to track a character's progress for DM's and players alike and can help players with knowing when to start planning for when their character levels (that way when they do level, they can just switch to the new stats/features and continue playing without having to stop and figure them out).  With Milestone based leveling it's harder for players to keep track of progress, but allows the DM to set the pace of their campaign without having to worry so much about planning encounters around their XP reward; plus, if you have a group that isn't able to meet on a regular schedule you can expedite their progress without having to wait months on end in order to meet the XP requirements.

In the end, it all depends on what works best for your table.  You may want to ask your players if they have a preference, but ultimately it's your call.

In favor of milestone leveling, it’s a lot easier when all of the PCs are the same level than when they’re different levels, and with XP that will happen when players have to miss sessions. 

yeah, it's a pacing thing. ultimately you get the same results but in different manners which lend themselves to different types of D&D stories.

Quote from Tim >>

In favor of milestone leveling, it’s a lot easier when all of the PCs are the same level than when they’re different levels, and with XP that will happen when players have to miss sessions. 

Only if you decide to do it like that. I would never allow a player to fall behind in levels just because they had to miss sessions, assuming the actually had to, and are not just blowing us off. (Blowing us off = you lose XP, no mercy there.) How you handle it XP-wise is up to you and can vary from game to game. But if the player misses and someone plays the character, then XP as normal. If they miss and their character also is not there, and therefore was "somewhere else for a time," you could either (a) play out a mini one-on-one with them and award XP, if you both want, or else (b) state that they had "off screen adventures" and give them an amount of XP roughly equal to whoever got the minimum while they were gone (maybe, minimum - 5% or something just to give the person who was present a little more credit).

I mean I understand the idea "if you're not there you didn't experience anything," but, assuming we are all grown-ups here with lives and commitments and emergency room visits to the hospital at 3 AM with a sick baby and things like that -- we shouldn't be using RL issues to punish characters (or their players, btw).

Again, if it's chronic you have a situation on your hands and may have to re-think... and if it's negligence and the person is being a tool-bag and just leaving your group high and dry, then no XP at all. And maybe worse punishments or if it's glaring enough, booted from the group. But for someone who missed a couple times because their kid was sick and there was no baby-sitter... I would not let them fall behind on XP over that, and I would hope I am playing with a group of players who feel the same way. (If not, I'm not sure I'd be playing with them...)

As per usual, BioWizard said what I was going to say only better and first. Adulting happens. Weddings, funerals, birthdays, work/school commitments, company Christmas parties, planned vacations to take the kids to Disney World, illnesses, even long awaited concerts all happen. That shouldn’t preclude us from having fun playing make believe with our friends. That’s really all D&D is, slightly “grown-up” make believe with rules to determine who shot first.

I only ever use XP, I love math, and aren't afraid of players being different levels. I like being able to give something to the players other than gold and treasure to reward them when they do something well.

Quote from LeviRocks >>

I only ever use XP, I love math, and aren't afraid of players being different levels. I like being able to give something to the players other than gold and treasure to reward them when they do something well.

 Me too

For a first time DM I would recommend XP based leveling. It's more to keep track of, but it helps to keep your players progressing at a natural pace. If you're not familiar with when a character should level up for balance sake, it can help to just have the natural XP growth the character's acquire through gameplay as a guide. With milestone you can find situations where the characters sometimes take too long to level up, or you might find the players level up too fast and combat becomes too easy.