We have stayed at both and the Poly is my favorite over the two for these reasons:
Poly Pros:
1-huge rooms, larger than the YC
2-More comfy day bed than YC, and a day bed in every room. No worries about not getting one
3-Getting MK took 5min tops on the monorail...so much fun. MK from YC was 35-40min. Seemed like we were always waiting for a bus to come, multiple resort stops and then finally on our way
4-Smaller layout for rooms closer to the lobby. We requested room on 1st floor close to the pool and lobby at the Poly and got an awesome room, steps to the GCH and pools. Our room at YC was far from the pool and lobby. Took quite a while to get there and was not convenient for refilling our mug or getting quick service food.
5-Captain cooks, open 24 hours a day. Good food and central location to refill our mug
6-walking to the TTC. Easy to walk to the TTC and get onto the EPCOT monorail
7-Buses to AK, DTD and water parks were quicker. YC area buses share with BC and can go to the Swan and Dolphin and Boardwalk before going to your destination. We ended up taking a cab a few times this year, never did that at the Poly
8-watching Wishes from your room, beach or pool. Not having to fight the MK crowd for a good Wishes spot was awesome!
YC pros:
1-SAB pool is awesome, much better than the Poly pool. We did not like the main pool at the Poly much, very small and crowded. The quiet pool was better for us.