I have stayed at the Somerset at Grace Bay and at Beaches when my son was little. It depends what you are looking for. The beach is amazing anywhere along Grace Bay so that is not a huge consideration.
Beaches is REALLY busy. This can be fun and there are tons of activities to do. But the food is included and is really mediocre, compared to some other places on the island. It is a great place with kids as there is non-stop action. We never left the resort as there was so much to do here.
The Somerset was much more laid back. We had a beachfront room with a full kitchen. The grocery store on the island is really good and comparable to a US grocery store so we enjoyed stocking some of our own food and cocktails. Non-motorized watersports were included. We rented a car here and explored more of the island and I liked that.
So - it depends what you are looking for. You will never see a beach much more beautiful then Grace Bay!