I bought yesterday Tamron 17-50mm and I can tell that it is very good lens. Acctualy, I changed my Nikkor 18-105 kit lens for it. And there is no need to compare that lens to any kit lenses that nikkor made, becouse, it is KIT. First of all, 18-105 VR is very dark lens and quite unsharp. It is good lens only if you have sunny shine day or you don't mind using flash. Tamron, on the other side, provide much more light for all weather conditions. f2.8 is very high aperture and just perfect number for travel and experiments. 17-50mm is perfect range for city and landscape as well. Tamron is solid, heavy, with great glass.
You have great body (d90), so even on the very grey day you can raise ISO even to 1000 and shot photos all day long without any risks that you will take "noisy" pics. If you take NIkkor 18-105, be prepared to hold ISO quite offten at 2000. Believe me, its very personal experience :)
My advice: take Tamron. It's more expensive, but very reliable lens. With Nikkor kit you will regret very soon.
Best wishes. Excuse my english