L.U. is for GTA 3. SA:MP is for GTA:SA. L.U. is newer and has better sync and more scripting freedoms than SA:MP from what I've heard.
P.S. I can't wait either! As soon as it's released, I'm going to try and get Argonath RPG to adopt it and use it! I've got a
whole list of supporters that want to see it happen!