I'd say the Poly sounds like it would be best for you. All the hotels are very decorated, the Beach Club is maybe more traditionally decorated, but I'd think an 8 yr. boy would appreciate the Poly's take on Christmas.
The Beach Club's pool can't be beat but the Volcano pool at the Poly is pretty cool too.
The Poly's Mono-rail access to the MK and walking access to TTC for the EPCOT Mono-rail definitly beats the Beach Club if you're going to spend more time at MK.
Everyone has their favorites but we preferred the Poly's Concierge Lounge food offerings. I think that the renovations on the regular concierge longhouse has been completed, so the rooms should be like new.
That leaves the price which also favors the Poly. You can't go too far wrong anywhere on-site, and either of your choices would be great!