Both resorts are great.
Desert Springs is a much larger resort and more spread out. The views overall are also better from the Desert Springs units, especially if you request ahead of time based on what's important to you. I also think the resort next door to Desert Springs is more impressive and has better restaurants to offer. The lobby alone is an experience (this is the lobby where boats come in and pick you up to take you to your reservation). The Spa at the JW was also recently renovated and redone, so it's impressive. If you are staying via getaway or exchange you have day privileges included if you stay in DSV I.
The lock-off rooms are different between phase I and II are different. In phase I they have two full or queen beds (I've never determined which they are) and no couch. In phase II there is a king bed and a couch. If a couple is use to sleeping on a king or they are large people, then the bed thing can be a big deal.
The Westin was beautiful too, felt a little step above Marriott in terms of luxury and I really enjoyed the full service restaurant right next to the t/s pool. You can also use the resort pool if you wish. This property was much smaller, not as many nice view rooms although I really liked the views overlooking the creek area. Very pretty. The unit layouts may be more conducve to two couples. There's the larger one bedroom with a king bed and full kitchen then the smaller lock-off unit with queen bed, small family room and dining table and limited kitchen (no oven). Don't know what the spa is like since I haven't used it. There is a nice cement walking path that stretches the whole length of the t/s and resort properties. Very easy walking right along the golf course.
Both resorts are equally close to golf and shopping. Both have pool areas that are very nice. Both have shuttle service between the t/s property and the resort next door.