More successful (assuming a sales-metric): quite likely, given a guaranteed advertising budget. But less of it (%-wise) would go back to Notch.
Better: depends on your metric. Fewer customers = less feedback = more likely to get your idea implemented. More customers = more employees = more code backing the game (overall better), and likely more bugfixes and better-performing graphics. More control in the hands of one person = more personality-driven design decisions = ? (and I wish to point out that Apple works largely this way with Jobs - though again, "better" is still subjective)
But, at an absolute minimum, being part of Valve will put more pressure on sales than there currently is. Even if they explicitly say "do anything, we won't even look", there's an implicit push in that direction. He's now made enough money to do literally anything he wants with this game, with no pressure to make more sales. I count that a purely good thing, because there is no game for everyone, and without pressure to appeal to more people it's more likely (IMO, also still subjective) to become better in its niche.