I've stayed at both resorts, but only at the Westin Resort & Spa before it began conversion; it was entirely a hotel at the time. So I have no experience in the timeshare rooms. Still, overall as a resort, I preferred the Westin Resort over the Westin Lagunamar.
Getting it out of the way, the rooms are much older and so is the entire resort. However, I really liked the beach and the overall feel of the resort.
IMO, if you're a beach person, you'd prefer the Westin Resort and if you're a pool person, you'd prefer Lagunamar. The pools at the Westin Resort (there are a least 4) are small and featureless. However, the whole resort is oriented to the beach in a way that Lagunamar is not. At Lagunamar, I felt cut off from the rest of the resort when I was on the beach. It somehow felt isolated to be at the beach and the beach never seemed to be busy.
At the Westin Resort, most of the loungers orient towards the beach rather than the pool. There is a whole level of patio at some of the pools that is built with sand and palapas that is just one level up from the beach. Then it is just a small level down again and many more chairs and palapas front the actual beach than at Lagunamar.
The beach is actively used. There were always a lot of people walking along it and it was swimmable. Since it so close to the rock outcropping at the Club Med the water is much calmer and provides a reef for snorkeling.
Yes, it is the furthest south area of the Hotel Zone, but we like exploring and we didn't mind the extra bit of time on the bus. There is an excellent seafood restaurant, Navíos, that is walking distance from the Westin, but not much else.