I wasn't going to post, but then someone came here just to plug for his review and throw around a ridiculous piece of advice.
There's no reason to get the Tamron 15-30 f2.8 for a DX camera. If you intend to put it on a full frame camera soon, that's a different matter. It will utilize approximately half the megapixels that the 18-35 f1.8 Art will on a DX sensor at f2.8. You won't see a "twice as sharp" image from an 18-35 Art, but the difference will be quite significant, especially if you crop at all.
The two lenses have wildly different purposes. On a DX camera, a 17-50mm f2.8 OS will perform just as well, and you'll get 20mm on the long end. The 18-35 f1.8 will beat out any of the competition on a DX camera in its focal range. The Tamron 15-30 does everything well on a DX camera, but nothing exceedingly well, and shoots in a very limited range. Get it if you plan to go FX, but that's it.