I've been talking about this on Twitter (http://twitter.com/elahti
) with folks.
If you're just experimenting, pick up the original Arma 2. Between free single-player missions (find them at http://www.armaholic.com
) and our dedicated A2 server (we have separate servers for A2 and Operation Arrowhead), there's more than enough to keep you occupied.
That being said, Arrowhead is the definitive version of the game. Its barren desert environment runs at a cleaner framerate, there's more and cooler equipment (thermal optics), and the single-player campaign is basically a response to the mistakes that the original A2 campaign made. Let me know if you have any more questions.