We have the 21 ltd and Tamron 17-50 although the Tamron 17-50 is new and we have yet to give it a decent work out. I've never considered the 21 ltd to be a particularly sharp lens, but I still like the pictures it takes. So far the Tamron is both really sharp and seems to control CA really well. But it's much bigger than the 21.
This is your 21 ltd. type of shot. A softish, flowing dreamy kind of lens.
This is one taken with the Tamron, a little sharper, a little more micro contrast.
Overall, I think the Tamron 17-50 is a little sharper, the 21 is more of a mood lens. It depends on what you want. These pictures aren't meant to be some kind of direct comparison. Just an illustration of what I think about the two lenses and where I'd use them.