DO NOT STAY N FIESTA HOTELS!
The rooms are tiny, salt water runs out of the taps, they are billed as family hotels, and therefore are not clubbing friendly.
By this I mean, you are not allowed to invite mates back to your room or even to use your pool. Imagine all the new-found friends you've made in Space and you want to take back but are not allowed to do so.
Also, and this is the worst thing. They have poolside entertainment for families, which involves some spastic on a loud microphone singing all day long. She normally starts around 10am, just about the time you've got in from Pacha and are ready to pass out.
AVOID AT ALL COSTS!!!
My advice is to hire a car and stay at El Puerto. You can walk to all the bars and clubs, but will need to drive to the beach. A bit of a pain, but the hotel apartments are the best in Ibiza Town. I've also heard Jet are really good, but haven't yet stayed there.