Hi
Yes, I have both. They are really really close. The Nikon is better at 17mm, at 35mm the Tamron is a tad better - at least in the center.
The only really difference between the lenses is build quality and cost, and corner sharpness where the Nikon does have the edge but in all honesty the Nikon isn't worth 4x the price in pure optical terms. Although overall the Nikon is the better lens. I did test these side by side.
For my Sony A900, I use a Konica Minolta 17-35 which is actually the Tamron 17-35 with a new paint job, the Tamron 17-35 is a superb optic and works well with D700 (I imagine it'll be OK with a D3X too as the KM 17-35 is certainly good enough for the Sony A900).