I am in full support of the Polynesian (even my profile icon was taken from there). Easily the best part is being able to get to the MK either by monorail or boat and not dealing with buses. It feels a lot more like you're on vacation when you don't even have to look at cars or buses. The tropical feel also feels very much like a getaway vacation to me. Overall it feels like an escape and it is very relaxing. The only thing you want to keep note of is there are many buildings there and some of them are pretty far from the main building, so try to get into a closer one and one that isn't overlooking a parking lot. Also note, even if you are not on the ground floor, you may not get a balcony, which may sound odd, but some of the buildings not all the floors have balconies, instead there is just a big window essentially where the sliding doors for the balcony would go.
Plus side being there is you can watch Wishes from the beach area, which is pretty chill and relaxing and offers a different perspective (I always enjoy seeing the different perspectives you can view Wishes from).
I am not as familiar with the Beach Club. However, what little I have seen of it it does seem to also be very nice. However, it's best if you prefer EPCOT and Hollywood Studios. Going back to the Polynesian, I prefer MK, it's the most Disney feeling park and has a level of escapism the other parks lack, so it's a big plus for me (and you can easily walk to the Transportation and Ticket Center to hop on the monorail to EPCOT). Back to the Beach Club, it is probably very nice. It really comes down to what theme you prefer and which parks you want to easily be able to frequent the most.