The big problem with the spray foam (aside from the stupid cost) is that as the gasses escape over time the R value will decrease. Basically the best you can get from air is R 4. So these R 7 foams will slowly make their way down to to R 4 over the course of 15 - 20 years. They are also pretty nasty, high embodied energy, they give off terribly toxic fumes when burned and they drive moisture into the framing lumber. Cellulose draws moisture away from the lumber. It's also made form recycled news papers. There is no end to the benefits of cellulose. The only thing that foam has on it is R value. Just make you wall a little thicker. It'll be greener in every sense of the word. The indoor air quality will be much better. It will cost less. You can't go wrong.
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