Need your opinion

I believe there's no difference, besides the Steam advantages (if you like Steam, like me). But both versions should work with the mod.

LOL, I misunderstood the post. Well, I was wondering the same. I have the GG version of Arma2, and I don't know if the Steam's version of Arrowhead works with the GG's vanilla game. Ask dwarden on twitter, he always answers

@FoltynD

Its easier to set up by buying CO...

If all you want is DayZ, all buying the base Arma 2 does is add some missing textures. Since Arma 2 is partially free on Steam, you can buy just OA and successfully play DayZ, just with missing textures. it's up to you if those textures are with the 7.50.

the non steam version and steam version are compatible if that's what you ask.