As has already been mentioned, the Prime has a lot in common with the 39gII. Especially the programming environment is quite close. It would be interesting to see a comparison sheet between the two devices. Can there be (non trivial) programs that run unmodified on both platforms (source code wise)?
What I'd like to understand is why there are two HP Connectivity Kits for the two, even carrying the same name. I've tried it, either kit only sees its matching calculator only: The Prime kit sees the Prime device and the Prime Virtual Calculator, while the 39gII kit sees the 39gII device (unfortunately not the 39gII Virtual Calculator). I'm wondering why HP didn't create a single Connectivity Kit for both (as the identical name already suggests). As an aside, TIConnect works with almost all classical TI graphical calculators from a single program.