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 into the perfect house; Floors are done and cutting up the cement for the new bathroom that wasn't actually plumbed like the seller said it was; 8/1/13: 30k & 400 hours into the perfect house that only needed new carpet and a coat of paint...
12/1/13: 35k & 500 hours I hope it ends soon
02/1/14: 37k & 550 hours and the inside may be finished soon.