I would suggest choosing Phuket over Pattaya anyday! You are right, the beaches in Pattaya are pretty lousy. Since, chilling at the beach is a major part of your plan, you should defiantly not compromise about it.

If you are worried about the Nightlife, Phuket actually has a really amazing night life as well (Almost as comparable to Pattaya). You can head to Bangala Road, which is the nightlife hub in Phuket.

I have been to both the cities, there's nothing you are going to miss out if you choose Phuket over Pattaya.

If you have more doubts, I have written a detailed answer on Quora about this, you can check it out - Phuket or Pattaya - Which is Better?