San Antonio gives you the opportunity to explore a lot of the bars etc with MUCH cheaper drinks! However this is not everyones cup of tea! Playa D'en Bossa is slightly quieter, however this is where you will find Space and Bora Bora, and if you find San Antonio too much then you might find Playa D'en Bossa more your scene, lots of people on this board seem to prefer it! Hope this helps, I am sure you will LOVE Ibiza no matter where you decide to stay! 
leap270 said:
......This will help you decide which resorts you want to stay in. If you are on a budget I would stay in San Antonio, you are close to most amenities here, and the buses are really cheap at night to go to and from the clubs. There is an abundance of hotels and apartments in San An to choose from, and you will find some on this website also! a lot of people recommend the Jet Apartments in Playa D'en Bossa, although I have never stayed there but I am sure there's plenty of info available somewhere on here! Most younger people stay in San Antonio, but sometimes these people are VERY young, if you know what I mean!! Saying that I am 23 and Im staying there, so really it's all about your own preference, in my opinion San Antonio is a good base for someones first time in Ibiza because it is such as good central base to explore the rest of the Island! 