Raspbian is currently going to stick at 32bit for now “At launch, we are using the same 32-bit Raspbian userland that we use on other Raspberry Pi devices; over the next few months we will investigate whether there is value in moving to 64-bit mode.”

so the rpi2 build will be the one to use for now. when calling retropie-setup script you may need to do

sudo __platform=rpi2 ./retropie_setup.sh