For swimming the Aston... for snorkeling the Westin , however they are close enough to walk easily to each other. Both are nice. The Westin is bigger and busier I would say. It’s a time share so you don’t get to pick unit other than category. The Aston MKV you can go through Aston and pick a category or you can rent through private party and pick your unit
The Snorkel Queen gave you great tips.
I find MKV nicer for the space and low density. But others love KOR for the full resort amenities.