I've stayed at both and like both. Are you going with kids and what are their ages? What are you goals for the trip? Will you be using the amenities at the hotel or will you be mainly going to the parks?
Staying at the Poly is wonderful. You're right on the monorail, have two great restaurants there, the beach, view of MK and the other monorail hotels. The pool is nice but not exceptional and there is not spa/hot tub. No food court but a great coffee bar w/pastries. There are more water actities available also (which we didn't use at all).
CSR is the moderate that is closest to the deluxes in amenities. It has a great pool with a hot tub. I think the grounds are beautiful and love walking around the lake. I've never eaten at the Mayan Grill but the Pepper Market is a decent food court (a little more expensive than some of the other food courts but has more selections).
We're staying at CSR on our next trip because we decided that we didn't want to spend the extra money on a deluxe. Instead we're doing several tours, La Nouba and table service dining. (It was a difference of about $900 for our 8 night stay in Sept. between AKL & CSR so the choice was easy).
If you really want the convenience of the monorail, can or don't mind the extra money for the resort, and don't care about the hot tub, stay at the Poly. If you have kids or teens that will want to spend more time at the pool and playground, CSR is probably a better choice. And I think a great choice for couples (it's really romantic especially at night!).
Forgot about busses - we rent a car but the last time I stayed at CSR we didn't and I had no problem w/ the busses from there.
Don't know if this helped. Look at the pictures. Do a search on the Disboard for other threads on these resorts. Buzz? and Epcotfan have great websites for CSR with links in their posts and Tikiman has an amazing Poly website.
You won't be disappointed, imho, whichever you choose though I think I convinced myself that choosing CSR was the best decision! Have fun.