I have not been to Penang, but I have spent a fair bit of time in Phuket. I would advise that you go to Penang.
Most of Phuket is very touristy, and Patong (the big town on the island) is the sleaziest place I have ever seen. It was interesting for one evening though.
In fairness, there are some beautiful places in Phuket. If you go I would recommend that you stay at the Amanpuri, the Banyan Tree or the Meridien Phuket Yacht Club. All are very nice resorts which are not infested with tourists. Make certain to visit Phi Phi island. In fact, if I were to go to Phuket again I would just stay on Phi Phi. It is spectacular.
HKF