They both do exactly the same thing, except steam does more.
And anyway, you can't very well go saying that Steam won't succeed just because it got a foothold first, because this is different from things like facebook and myspace. Anyone can pick up a profile and move on to another networking site, but you can't very well just move onto a new distribution platform after you've bought all your games on steam. You'd be giving all those games up. In a case like this, as long as the games you own on steam are locked exclusively to steam, their head start pretty much means an easy victory.