Dragon Rising is a mainstream shooter and feels a lot like Battlefield and not like Operation Flashpoint. so if you are looking for a real sequal to Operation Flashpoint, then DR is a no-go.
You can just see it was made for a console.
Personally, ArmA 2 all the way.
Better controls and more movement options, less clunky except inside buildings, larger scale, better-looking overall, better modding capability, no size-limited maps in multiplayer.
Then there is the option to control several squads, or even Warfare, a kind of RTS mode where you can control an entire army.
ArmA 2 has more content, and does way more gameplay wise.
That's not to say it's without problems, because with an open-world sandbox game like this there will always be unforseen situations which might screw with the game a bit.
Just make sure you have a rig that can handle the game.
That's the only thing DR does kinda right, in that it performs better then ArmA 2. At the same time the graphics are crap, if you look past the damn HDR filter they put over it, coloring the entire world in either orange or light-blue which i guess should resemble the night. Also, the radio voice-overs are less robotic, i'll have to admit.
Other then that, DR is underwhelming in every way.