In my opinion both resorts are a very good choice, next time staying in phuket will not be easy for me to make a decision.
If you don't like mega-resorts, the Ren is definitely the better choice -- especially if you're willing to rent a (cheap) car at the Phuket airport. The Ren is arguably more isolated without a car. When we stayed at the Ren, we drove over and took a look at the JW Marriott. We were so glad we picked the Ren. And if you have status, you're likely to get upgraded to a plunge pool villa at the Ren: it would be very hard to beat that at the JW Marriott.