Yes, Dez is a better receiver than Williams. He makes contested catches, this isn't a news flash. And when you say "He catches a ball in traffic", when? I think you're using that phrase wrong.
As for Williams not making contested catches, that's not the kind of receiver he is. He's not a jump ball receiver, he's a space receiver. You know, there are more than one type of receiver. He's a route runner (Still improving), he's a deep threat, he makes defenders fear Romo going over the top. You say he makes Romo's game a lot harder, he doesn't and you haven't given proof of this. He actually makes it easier by stretching the field, something we didn't have since Owens left.
And you for some reason think Romo is going to stop the scrambling and improvising because he has a similar WR to Bryant as our #2, he won't. Romo improvises when the line of scrimmage breaks down. not because of Williams. It's best for Romo to have more than just Witten to throw to when he does his Houdini act, because Dez Bryant rarely finds a way to get open when Romo is doing that.