Both Rapa Nui and Tahiti are closed. They are being refurbished into DVC units.
The resort monorail to the MK only has one stop (at the Grand Floridian) between the Poly and MK. It is a quick ride and far less crowded than the express monorail from the TCC. Also, if you want to attend morning EMH at the MK you need to ride the resort line as the express won't be operating.
There is also a boat from the Poly to the MK.
The Poly is a very small resort and you can walk anywhere (transportation, food, pool) in 5 minutes or less. If you want a standard room that is centrally located, request Rarotonga.