Personally, I wouldn't do either... I'm there on this date and think we are going to dc10 and cocoon... however... back to your trip...
My best friend had her first trip to Ibiza in July, and I advised her to go to Tiesto at Privilege.
She loves Tiesto and I thought, from what I have heard to see a club like that filled and buzzing, with a DJ you love would be amazing. She wasn't disappointed and had a fantastic night.
I would skip SHM. Pacha packed is a nightmare, and check out Tiesto. If your into that sort of music you will more than likely have a great night.
Apologies if I'm rambling, not long got up...
8)
Have a great trip...!