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.