Recruitment, Roles and Intelligence

Hey, first off sorry if this isn't the best place to post this but I've got a few questions about some of the roles and recruitment for the Royal Artillery.

To begin which just what's some of the main differences between 148 battery (29 commando) and 4/73 Sphinx Special observation post battery? I mean aside from the fact that 148 are commando trained and work alongside Royals it seems like they perform a pretty similar role (perhaps different environments is the only one that seems obviously different to me) - does one receive more jobs than the other? is one harder to get into? does one have more opportunities? any chances of getting contacted by the enemy higher in one or the other? just anything would do really cheers.

Second question I'm wondering if someone could outline the difference between the batteries above gathering intel with reconnaissance, surveillance and all that compared to an OPMI in the Intelligence corps? For sure the OPMI I'd think are more likely to analyse and stuff but is there some main difference in how they gather the intel or how close they get to an enemy and again I guess any differences in the chances they'll get contacted?

And then finally just about recruitment in the Royal Artillery - following phase 1 (and maybe phase 2) do you then just have to give a preference about which regiment you'd prefer to join and then the army will just bounce you wherever they need you or is there some for of guarantee you can get that you'll end up in the regiment you want?

Thanks for any help and sorry about any errors or whatnot - Cheers