I have all three games, but if they release it on Impulse or GamersGate, then I'll buy AoW:SM for a second time -- in the vain hope to encourage the publisher to finance a sequel. Naive, perhaps, but the AoW series is one of my favourites and there's little else I can do to help the chances of a continuation.
Steam is better than no digital release, but yes, I'd definitely prefer Impulse or GamersGate.
Edit: Just saw the post about it also hitting Impulse. Good news!