We have 3 ds and have stayed at both several times. My DH prefers the beach club, the rest of us prefer the poly. They both have their good points. I agree wih the other posters of where you will be (parkwise) and also how much time you will spend at the resort.
Besides transportation, pool, onsite childcare, restraunts etc. which are equally, although I think the beach club might be a few dollars less for a standard room.
The probelms we had at beach club were:
Lack of quick place for snacks and mug refills
During a bad storm (we usually go in summer) you are pretty much stuck there
Poly: captain cooks 24 hour snack place for food and mug refills. The grill closes at 11 I think. Once we got the last pizza. This did not go over well with the 3 ds.
If you have children the free childrens activities are excellent, pool games, arts and crafts in the neverland club and arcade challenge etc.
My ds prefer the slide at the volcano pool versus stormalong bay. and its a hassle to always have to show your room key to get into Stormalong bay and get a towel etc at the Beach Club.
The beach club does have a cute table in the room if you are stuck there and you can get chess and checker pieces from the front desk. In Aug. they had a tornado warning and shut down trans. for a while. The water sports are equal at both , example water mice, bike rental etc. I do think the Poly is a little more kid friendly. You can enjoy the beach at night and take in wishes and the water paegent. Awesome. Sitting on the lounge chair, looking at the castle. I do know in the summer it gets pretty crowded in the pool and lounge chairs are scarce directly at the pool during the hottest times of the day. Good Luck deciding.