Well, the biggest problem you will have is that when you get the 21, you wish you had gotten the 20-40. Ad when you get the 20-40, you wish you had gotten the 21.
But seriously, both are superb lenses. I do use the 20-40 more often than the 21. As it is just a bit more versitile. But if I want to keep my gear small and light. I will use the 21. (and put another of the limteds in my pocket. Could be the 40, could be the 15 or 70)
So I would say, you want to keep it small and light, get the 21. You want a bit more versatility, get the 20-40 (you just might realize, that you will start using the 40 less)