What time of year?
The SAB mini water park is a huge part of YC/BC (it's why I decided on YC over Poly this coming August for my DS9). You kids are the right age to get the most out of it. To counter, Poly offers views of the MK fireworks and water parade from their pool and beach. The one major downside is the pool of the Poly seems relatively small and some complain it's "people soup" in peak times.
Poly offers easy access to two parks...MK and Epcot...monorail, boat or ferry (from TTC) to MK, monorail to Epcot (you can skip having to ride two monorails to Epcot by making the 5-10 minute walk right to the TTC. Kids love the monorail, and it's probably easier on the grandparents was well. You have to bus/drive to DHS or AK.
BC offers boat access to Epcot and DHS. You can walk to either park as well, it's an easy walk over right into the World Showcase, we like it for easy assess to dinner options. You can walk to HS, but it's not a short walk. For AK and MK, you bus.
Rooms are bigger at the Poly. I prefer the Hawaiian/tiki theming of the Poly (maybe my love for tiki goes back to staying at the Poly in the '70's), but others find it tacky. YMMV.
BTW, the choice may seem hard but there is no wrong option between the two.