How can you make a MTA vs SA-MP topic without knowing what MTA is???
MTA is, as others said, another Multiplayer for GTA. MTA

A Race owned, and MTA DM was looking promising, and I stayed patient waiting for it for 3 years until they released it on 3rd Jan 2008. Then everyone realised how crappy it was, despite taking 3 years to make, most of which was spent bug fixing.
SA-MP on the otherhand released quickly, without a Race mod, and the sync wasn't all that great but at least they released and kept everything as promised. They also released new versions with nice bug fixes and stuff.
I was in MTA's BetaTeam (Peter too) which caused me to come to LVP. BetaTeam members had to have irc, which I got, and learned to see other people's channels, looked at Peter's first, and found LVP channel, and found LVP's IP and just joined, and here I am! So if it wasn't for MTA being so crappy, you may not even know who The Munch is, even though I've been on SA-MP since it came out.
So to sum it up:
MTA DM = Scripter's paradise (LUA, not crappy PAWN) and also people who are into a wide range of gamemodes (Some of which are actually cool, for example, Hay, Fallout, and Interstate 69).
SA-MP = Freeroamer and Deathmatcher's paradise (MTA DM had promising sync, then everyone found out the sync wasn't all that great compared to SA-MP, except car sync). And possibly RolePlayer's paradise, although with MTA's scripting advantages, I guess MTA would grab an RP fan's attention.