Mostly a difference in power and that the air gets somewhat warmed (and there is a foam filter that removes any possibility of picking up particles that get shot at the paintwork.) A leafblower will do the job too. The larger models are darn powerful though, I think the top of the line model has something like 8 HP... but cost starts to get prohibitive.
Still requires you to scrub at the bike with a rag, though. Plus it takes much longer. And you can't get into cracks and crevices to get the water out of those. I've done the chamois thing and that is exactly why I went with air instead, so much faster and more effective.
Should have put this in the first post, I guess: