Assault on Black Reach is meant as a demo to the game as a whole. If you've seen the game and know you want to play, go with the army that seems coolest.
Before you buy any models, I advise people to talk with other players (like you're doing) and figure out what playstyle you want (shooting, assaulting, lots of tanks, lots of infantry, etc.). If you have a list in mind before you buy some models, you'll be better off for it.
And yes, most of the activity on the forum these days is from the Plainville store, where there are a great number of Tau players and a handful of Space Marine players. The Abington store is a bit more evenness in what people play. There's just something in the drinking water in Plainville; maybe it's because the store is young and vibrant like the Tau.