I think you should go for Combined Ops. This includes ARMA 2 and Arrowhead, so you'll be able to enjoy 2 whole theatres of war, and benefit from the fan made content avaliable for each game. Vanilla ARMA 2's Chernarus is your typical former Soviet-bloc eastern European country. Arrowhead's Tajikistan is more like a central Asian Republic.
Don't buy it if you're more into COD/Console toy games. The learning curve is relatively steep, (although not as steep as a true milsim) and you might find yourself fighting against the relatively unpolished interface for a bit, but once you get past that, there's no comparison really.
