I'd say go for ARMA 2 + Operation Arrowhead (ARMA 2 Combined Operations)... get the two content expansions if you want extra content (British Armed forces adds more than Private Military Contractors, but I picked them both up on Steam during the Xmas sale so that I could play the latest Flashpoint dynamic missions.)
Be sure to hit up the official forum (http://forums.bistud...display.php?f=9) and Armaholic (http://www.armaholic.com/) for all of the latest & greatest user-based content (maps, mods, missions, etc).
If you have TrackIR or FreeTrack then you'll have even more immersion.
I'll be honest, I really didn't play the campaigns or the stock missions as there are tonnes of user content available that always keeps me entertained. I've also been known to spend hours just tinkering in the editor .
Well, that's my $0.02 for whatever its worth.