Do the spray foam.
Trying to bend rigid Styrofoam into the curve is a big pain, and you will never get as good of R value as the spray foam due to gaps, cuts, ect.
Get at least 3 quotes for the spray foam if you can. Or just call around for the price per Square Foot.
"Don't argue with stupid people. They will just drag you down to their level, and beat you up with experience."
Patently waiting for the apocalypses to level the playing field in this physiological game of life commonly known as Civilization