If it’s going to be a paid game, I might suggest waiting for the whole game to be done, then releasing the full game (paid) along with a demo on the page (free).
If it’s a free game, I might suggest releasing a demo first.
Of course, there’s multiple schools of thought. And it’s not really the case where one way is guaranteed to bring more traffic or interest than another.
Couple of reasons, really:
The game might still change after the first demo release, creating a little confusion. Confusion which generally would be more likely to be dismissed if a free game (in my opinion).
While you might build hype initially with a first demo release, the hype will tend to go down over time. With the potential loss in hype, it might take good communication (and a bit of extra work) to keep people who would want to buy the game interested until the full release.
