These are two totally different resorts. La Casa is small, rustic and with really warm, caring service, whilst las Ventanas is a world-class, very fashion conscious resort, which appeals to the Beverly Hills crowd. I like both, but for very different reasons. You have to decide what sort of experience you crave.
I am not keen on the rooftop terrace juniors at Las Ventanas, preferring the lower floor juniors, as you can step right out to the pool, which winds around the front-row juniors. I find you don't tend to use that upper terrace in the rooftop juniors, unless you are really into hot tubs. The spa at Las Ventanas is very good indeed, as is the food. The beach is essentially unswimmable, due to rough water, but it's very attractive and great to sit on.