Our first stay, when our kids were about that age (2,5,8) was at the Poly. We all loved it. We are going back there this year for our 8th trip to WDW. We have stayed at the contemporary, for 3 nights around NYE in 2003. I agree with the previous poster; we had a tower MK view and the fireworks at midnight were absolutely awesome. We could look on TV and see the people like sardines at MK, and we were so glad to be watching from our room. I think CR would be great for a big fireworks occasion. That said, I don't think I'd like a wing room after being in a tower room.
The Poly does have a way better pool--that was one of my considerations this time since we are going in July. Also, the kids picked it over the CR (they will be 8,11 and 14 this trip--waah, they grow up so fast!).
I think you might want to stay at the CR before your kids are grown and "jaded" about it--when we were there DS was 5 and just fascinated by the monorails--but you can't go wrong with the Poly either.
Robin M.