I can't say anything about the brand of equipment that you are looking at, but, in the New England area costs are about $26 per million BTU for electricity, and about $6- 8 per million BTU for either gas or oil.
This means that you will save roughly 2/3 of your heating bill. If you live in the mid atlantic area, it may not save enough to get a quick payback, but up here the savings will totally offset the costs in about 2 years (the more energy you use, the greater the savings).
As far as insulation, if you have 6" in the ceiling, to reduce the heat loss in the ceiling by the same amount you would have to go to 18". Of course it is hard to do that in the walls. But on the flip side, you could add more insulation for very little money, so the payback would be quick