I had the very good fortune of being able to do my own lens testing, admittedly not professional, but it is all I could do. But since this is how I generally view my photos, it was practical.
I found a camera shop that had both lenses in stock and I took some side by side pictures with my own camera, set on program, then took my camera home and downloaded the photos to my computer.
I took pictures of displays on a wall inside the store, flowers outside the store, and pictures of my wife both inside and outside. I used inside/outside as I thought the lighting might make a difference. It did. I took the same pictures at focal lengths of 18, 50, 100, 150, 200, and just for curiosity, 270.
When I viewed full size on my screen, I could discern no real or noticeable differences except the colors in one or two shots were slightly brighter with the Nikon. The different focal lengths did not seem to make a difference. I did see differences when when I enlarged and cropped. I did not expect what I got.
Here is where I show my lack of technical knowledge. In order to keep things as consistent as possible, I enlarged by hitting the button on my computer 4 times. I have no idea how much this actually is, but my goal was to go big and stay consistent. Hitting the button and using this setting was about the most I ever do, so I thought that it was valid.
For the flower pictures, detail and color was extremely sharp with excellent contrast and they were comparable to each other at all ranges except 150 and 200. Surprisingly, the Tamron was actually slightly sharper than the Nikon, but only very slightly.
For the outdoor pictures of my wife, I enlarged tot he point where all I could get was her eyes, eyebrows, and hair. Again, both lenses were comparable up to 150 & 200. In this case, the Nikon had slightly sharper images, being able to see better highlights on the hair and individual hairs were more discernible.
I saw greater differences in the indoor photos. I used only the indoor store lights as I didn't want to introduce another variable of the strobe.
I was at a distance of approximately 70-80 feet from the wall. At normal viewing, the photos were the same but again, the Nikon color and contrast were just slightly better, and you had to really scrutinize to see any difference. My wife called it nit-picking and she didn't see any difference. Maybe, because I am admittedly a Nikon fan, I wanted to see more difference than there was. In reality, it was a subjective call and I admit to being partial.
I enlarged the photos to the point where I could read individual letters on the tags on the merchandise, and this was a considerable amount. More than I ever would normally do, but I was enjoying the testing.
Once at the extreme enlarging, the Nikon really showed better clarity and less fuzziness on the individual letters on the tags, but again, while noticeable, the differences weren't huge, but they were there. Neither was incredibly sharp, but I think the Nikon took the lead here.
At the end, what surprised me most was that there wasn't more of a difference in the two lenses and that in a couple of instances, the Tamron appeared to perform better.
The Nikon did focus faster in most cases, but the Tamron could never have been called slow. It was more like one was really fast and the other was incredibly fast.
The Tamron was definitely lighter and that could make a difference if you're going to have it hanging off your neck for very long periods of time.
I never had any problem with either lens as far as creep or any mechanical problems.
Something I did not understand is that I saw veery little difference in image coverage between 200 & 270. Yes, the 270 did bring you in closer, but it was only slightly. I expected a more noticeable difference.
The price of the Tamron was slightly lower than the Nikon and the overall performance was really quite similar, but slightly better with the Nikon.
If I hadn't been such a stickler and gone to enlarging more than I would in reality, I probably would have been happy with either lens.
You can easily judge the character of a man by how he treats those who can do nothing for him.