re: Spray foam insulation

As other poster said, make sure to get a variable speed AC and variable condenser. Rather than 500sqft per ton it will probably need to be closer to 700.
I went with rafters sprayed. Make sure to check depth and for light. Should be no light from outside getting in and make sure they spray an inch on the rafter itself. They need to be sealed where the heat transfer won't condensate them.
Cost wise it costs a lot to do the rafters. There is over double the sqft than tradition ways and the insulation is about 4x more than fiberglass attic ins. That is probably why u have heard some foaming the joist rather than the whole roof. It will give you a better seal than the fiberglass and a higher r value, but is more affordable.