If you've never been to Phuket, I would say consider the full 6 nights in Phuket and then think about doing one or 2 nights closer to Patong. Perhaps Karon or Kata beach. I just stayed at the Renaissance last week and while it's a lovely property with generous upgrades, it's far away from all the action and it's really expensive getting anywhere. Of course this may change if you rent a car.