Eh, you don't miss a whole lot from BL1. If you had to pick one, go with BL2 it's an improvement in pretty much everything. Except maybe the way the story is presented depending on your views. In terms of the main story, BL1 was mostly about you trying to find treasure on a lawless planet with bandits/etc. and some humor here and there while BL2 pretty much threw humor in damn near everything. Not saying the latter is bad, but I know some people got tired of the jokes and whatnot.
Though from the gameplay I've seen so far, the Pre-Sequel seems to focus more on BL2 than anything in BL1.