We stayed in Tahiti last year and the views were great because it's right on the beach. It is very quiet and pretty there, but it is the furthest from the GCH. We were there in July, and the walk probably seemed longer because of the heat. In Dec. the walk might be really pleasant, and you have a beautiful quiet room with a very private beach.
As for the Monorail question, you have to look at the map of the different Longhouses. Some are closer to the GCH (resort Monorail) and some are closer to the TTC. When we were in Tahiti, we were literally next to the TTC so we used that one. It was very convienient to the boat to MK as well.
Truthfully, I don't think you can go wrong at the Poly. All the Longhouses are nice!
Good Luck!