I don't have any insight on specific heaters, however...
Insulation, insulation, insulation
A couple of summers ago, I added 12+ inches of blown cellulose type insulation into my mother's town-house attic. It has since paid for itself numerous times (summer and winter). It wasn't difficult at all, seal up any holes with foam (in a can), install some new soffit baffles (keeps the insulation out of the vents), and then spend the better part of an afternoon with a helper blowing the stuff in.