Personally, I would buy the game and install the mod. The Standalone will be a true alpha (because it's being worked on by a small, indie team) with textures missing, no vehicles, etc. The main thing that is being worked on is server-client architecture, which seems to be one of the only things that will be finished upon the arrival of the SA. I think that the first release will be missing a lot, and it's therefore a good idea to get the mod and Arma II