Only differences are cure time and expansion (or effective R value). The lower the R value the less insulation, the more expansion the lower the R value in the same space. I've spent ALOT of time learning about this. 3M won't REALLY help you figure out what you need and they'll disclaim the hell out of it, but you can use the 40% expansion and get >R6 in one inch which isn't too bad. There are some good $100 2 part foams that are very low expansion but expensive and harder to use.
10/1/11: Bought the perfect house;
11/1/12: 10k & 200 hours
08/1/13: 30k & 400 hours into the perfect house
12/1/13: 35k & 500 hours
02/1/14: 37k & 550 hours
08/18/14: 38k & 650 hours it's starting to come together or maybe I'm coming apart