Hi,
The nice thing about foam for an installation like this is that you can fill the rafter bays full of foam, and it has a high R value per inch. This gives you a good total R value even if the rafters are not very deep.

With conventional insulation, you have to leave a ventilation channel between the insulation and the roof sheathing, and this coupled with the lower R value per inch makes for a much lower total R value and more heat loss.

The foam needs to be the right type, and I believe that you want the closed cell urethane foam that is about R6 per inch. I believe that Corbond is one of the outfits that makes this stuff.

The foam is definitely more pricey, but in your case it may be worth it.

You might want to post the same question in the Finehomebuilding Breaktime forum -- lots of insulation pros there.

In my (probably biased) opinion money spent on things like this (good insulation, better sealing, window treatments) will payoff better than money spent on geothermal heat pumps.

Gary