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.

[Tozmeister]

Just make a save and don't delete it. Then you can play through one way and if you don't like it return to the save point.

Or better yet just play through with different characters taking different desicions.

[Inverted]

Playing through with different decisions is exactly what I'm doing, but seeing how I was going for the main story missions first this time, I would be a bit upset to find out 10 game hours later that I managed to lock myself out of whole side-mission lines.
You know, I really like this game but at missions it can be so seriously uninformative sometimes (but at least not always, like BE5).
There's that other mission too, where you only get told that you wont get it cause you don't seem like you could handle it... But what? How hard would've it been to make that dialog line "Sorry pal, I am looking for someone with more [whatever]". In fact I still haven't managed to find out why I won't ever get that mission.

Anyway, I decided that I will just keep forgetting to report back "immediately" and let Tanya live a few more days. It seems they don't want to execute her before I make my report.