We stayed at the Poly last month for a week - first time to WDW - and thoroughly enjoyed it. From our experience, I'd say the resort has 3 basic strengths (not necessarily in this order)
1) Location/ ease of transportation - walking onto the monorail to get to MK was just TOO easy - and it only took a few minutes to go either way (helpful when everyone's tired!) Also just walked over to TTC to catch Epcot monorail. Availability of boat to get back and forth was also a nice change. Bus service was fine too. We liked the fact that we had choices in transportation and they all worked well for us!
2)Ambiance/theming - definitely a personal preference kind of thing, but this definitely felt like a relaxing resort to us with the lush vegetation (even after Charley left his calling card).
3) Size of rooms - spacious is the best way to describe this! Again, this may not matter to you, but even though we were there w/ only 1 child, we liked to spread out and sleep in a large queen bed. These are definitely larger rooms than WL.
The volcano pool was a highlight for our DS (9), and he found enough to eat at Capt. Cooks (the convenience food place) when he needed to - and he's the ultimate picky eater...
Don't think you'll go wrong with either - it's a hard decision to make!
Have fun! (and sorry for the long post)
Beth