I tested yesterday these 3 lenses in comparisson. I will post some samples in the next days,now I'll talk about the general impressions.
The 1st thing I noticed when I picked up the Tamron was : sturdier lens! The build quality is better than my 18-105,it's smaller & heavier. The AF noise is not that intrusive,but I tested it outdoors. Indoors,in more silent scenarious,it could be a problem.
I compared F8 photos with both lenses. Observations: the Tamron is sharper,has less distorsions,has better color rendition & controls CA much better. (but still visible)
Took a step further & compared F/2.8 photos from the Tamron with F/5.6 from the 18-105...the Tamron still looks sharper. On another note,the Tamron bokeh is very pleasing...not expected that,even if the reviews I saw were pretty conclusive.
Now for the bad: the Tamron hunts a lot & AF is pretty slow (& this was outdoors,with a bright overcast light). This sample had backfocusing problems as well,but the point is I find this lens way to slow & unprecise for most of my work. (even portraits...I want to concentrate on improving my composition & comunicating with the model,not to worry about the lens)
Sorry Tamron,amazing potential,but...then again,it's a personal taste. Maybe if I had a D300,things would be different. My experience with this lens' focusing is only on my D80.
Next in line,the Nikon 35mm F/1.8 : like the previous 2 times I tested it,it's amazing. Compact,well built,metal mount,fast focusing (but not super fast),silent & SUPER sharp. Unlike my 85mm,this lens encourages me to take shots at F/2.0 because there's so much detail present. And the bokeh is amazing,if you know how to keep distance between the subject & background. (portraits-wise) Besides,it's a 35mm,it can't do wonders in terms of background compression. The CA is pretty well controlled & the distorsions are decent,it can be used for portraits if one is careful. Of the 3,the Nikon 35mm has the best color rendition.
All in all,I'll buy the Nikon 35mm on my next paycheck,because it's outstanding quality even at large apertures,it's compact & pretty fast,perfect for street photography.