Ibiza town is a beautiful and ancient town full of very nice places of interest. Imyself have not stayed here but have visited it a couple of times during my my 6 months working there. Never a dull moment in island capital which is known as the millionaire's playground and it would not be out of place to see expensive sports cars parked up and yachts in the harbor. All ranges of cafe's and restaraunts all serving delicious food.
San Antonio is a different place all together, with hotels ,bars and night clubs. The west end is an area for drinkers looking cheep drinks and a good laugh before heading to the clubs in the area. About 10 minute walk from the west end there a hand full of "chill out" bars on the sun set strip, the famous being cafe del mar and mambos. I myself foun savanna being more relaxed and less commercial but all of 5 or 6 bars are all together and I highly recomend the sun set from here at least once before you leave. Living there I rearly missed a sun set and never missed one on a holiday, it is stunning. Along the shore line there are bars equally as good for the sun set but they are great for sun bathing during the day. A couple of euros to hire a lounger and surrounded by gorgeous people coastline bar, sun sea bar and kanya are the best places to be during the day if you are not still partying.
I myself only visited San Antonio bay a couple of time postering so I really do not know very much about it but I do know from friends that it is very nice area with restaraunts and bars and a great deal nicer, cleaner and ultimately quieter but at a cost. It is around 30 minutes walk minimum from the west end but it is a nice walk with beach bar along the way and at night there are sand sculptures which are very impressive and funny.
Ibiza is a very beautiful island with amazing beaches all around it so hiring a car and driving off is highly recommended as we did this a number of times. One memorable time we drove to the beach to look at es vedra which is a little island opposite Cala d'horts bay. It is a long walk from the road and the road is very rough but it is worth it.