The effects of joining the army/rebels

[Inverted]

I've played through the game multiple times, with both vanilla and BSM, but there's one thing I could never figure.
After the prison mission with Tanya, on either side you get the chance to officially join them. So far I'd always turned them down because the ability to access their supplier (both sides have only one vendor if I'm right) is just not that tempting to be honest.
But now that I'm again at this point, I thought maybe I should finally get to know what is the downside?
You get told that it means you won't be able to do jobs for the other side, but at that point that would be impossible anyway. What I was always more worried about are the side missions, are those affected too?

In short, is there any downside of accepting their "invitation", or it's basically just a uniform given to you?
Will you get missions you would not get otherwise?
And do you have to wear that uniform? (I doubt, but who knows)
Do you get paid for real? (not that money would matter at that point, but mercs don't do anything for free)

BTW, it's kinda funny that I'm being offered the rank of army captain while I'm standing there in an Algeria elite colonel uniform.