san antonio pluses:
- sunsets
- lots of bars with OK prices
- a lot of nice beaches nearby (even if the main san antonio beach is crap)
playa d'en bossa pluses:
- bora bora, space, sankeys and ushuaïa on your doorstep (and ibiza town also just 5 mins taxi away)
- nice long beach (especially the southern part is still lovely)
both resorts also have their downsides, but keep in mind ibiza isn't that big which means that even if you stay in san antonio, playa d'en bossa is just 20mins away by taxi (and vice versa of course)