Hi,
San Antonio as a resort is cheaper; there is the west end for cheap drinks before big nights out and if money does run tight you have Eden and Es Paradis on your doorstep.
However Playa d'en Bossa is closer to te clubs as a whole, Space and Ushuaia are on the main road with Sankeys just around the corner. DC10 is also closer as is Pacha.
Privilege and Zoo are closer to San An and Amnesia is much the same distance from both.
If you stay in San An you need to allow for taxi fares: (These are rough prices)
Amnesia - 16€
Pacha - 24€
Space/Ushuaia/Sankeys - 28€
DC10 - 28€
Privilege - 14€
There is also a Disco bus service from the San An bus stop - behind the Egg roundabout - from midnight which go to most of the clubs; be sure to get the correct one, one goes to Pacha-Ibiza Town and one goes to Amnesia - Space etc from 12.15am. These busses run until 7am and you can get it back to San An too. (3€'s each way)
If you stay in San An check out: Pisces Park, Marco Polo 1 or 2, Sol Bahia, the Gran Sol, Casa Maria Apartments, the Roca Hostel and Hostel Ferrer and the Orosol Hotel & Apartment, the Blau Park and the famous Ibiza Rocks Hotel with live concerts included.
Reference -
http://www.ibiza-spotlight.com/night/san_antonio_i.htm
For Playa d'en Bossa check out:
Hotel elgarb, Ebano, Jet, bon sol (1 or 2), Es vive, Ibiza Sun, regina park, squash apartments
Reference:
http://www.ibiza-spotlight.com/night/playa_den_bossa_i.htm