It'll be out before September 30th, unless they delay it past the current quarter.
As for the question, sorrta, kinda maybe.
I'd only really play Diabo III becuase I'm not playing Starcraft II. I don't play Starcraft II becuase I'm so rusty and don't have the time comitment to keep up my skills. Diablo III doesn't have any of the massive amount of time, investment and skill requirement as Starcraf tII so I may play it in the coming months when i have limited time.
So I'll play the beta if it comes out, but if I dont get in I'm not going to shed any tears.