You'll be heating less surface area and volume.

In my area (zone 5), ceiling R-value requirement is R-49. The job would require one run of tunnel vents at the eaves, some fiberglass batts to block them up and 18"-20" of blown insulation on the flat ceiling. To get the same R-value on the rafters, you would need either a hybrid of spray foam/batts/rigid board insulation, or build down the rafters for batts. You'd also have to apply R-19 to the gables, and consider a venting solution.