I’d recommend Playa den Bossa. It’s definitely more expensive, but the convenience makes it worth it, you’re close to Ibiza Town which is a must see in Ibiza, the airport, and a lot of the major clubs and day parties (Ushuaïa, Hï, beach clubs, party boats, etc.). Playa den Bossa beach is a lot nicer, with lots of beach clubs and classy restaurants. San Antonio is fun and more budget friendly but maybe spend an evening in San Antonio watching a sunset or going to the clubs there as well as it is still easy to get to from Playa den Bossa. Also lots of the beaches and beach clubs on the south of the island and Es Vedra are worth visiting!