I would probably go for the Hyatt, but not really for the reasons mentioned above. Hyatt is a better deal, since it costs 15k GP points (which is what we used) versus Westin's 12K-16K points, since they just up'd it to a cat 5. Since it's much harder to earn spg points, value-wise, Hyatt is better. I have stayed at both properties 2x and both times, Westin upgraded us to SUPER nice suites. The first time was our honeymoon where we rec'd a huge suite with dining table, living area, etc. Most recently, we received a smaller suite, but still pretty nice oceanfront with 3 side lanais (separate bedroom). Now, or course, this cannot always be expected. The Hyatt did not give us that great of a room this past time...I had to beg for a PARTIAL oceanview room, but was given RC access. The partial view room we got was in the Smoking section, which we didn't want either. It was in another tower even, so it made the RC a bit further to get to.
As far as the pools, we liked the Westin better, just b/c the slide was more fun.
However, seemed as if the Westin had more kids. The Hyatt only has 1 hot tub too.
The spa at the Westin is pretty nice...free access for Platinums. At the Hyatt, even Diamonds don't get in for free.
Of course, the Hyatt had a decent RC, and it's always nice to have the option of snacks, breakfast, etc. Westin gave the standard $10 F&B credit, free movie, etc.
So, value-wise, I would probably go for the Hyatt, but we probably enjoyed our stay at the Westin more.