It all depends on how much you want to use it and how involved you will get with Macro. At a third of the price of the Nikon, the Tamron would give you almost exactly the same picture quality apart from some marginal differences in the corners of the images. http://www.flickr.com/photos/csaveanu/2739044725/
If you plan on getting heavily involved - most Macro enthusiasts will all recommend the Nikkor for things such as fast Autofocus and possibly being a more robust lens and not least for carrying the Nikkor Label. For the odd bug and flower at irregular intervals, no lens on the market will beat the Tamron for value for money. So what is your involvement like? And then - the lens is only one small part of Macro - it takes a LOT of experience to take really great macro - probably one of the most tricky types of photography to do well. And thus your real investment would be time and interest more than just dollars :-)

Hi Jimmix, I had the exact same issue as you are having with back focus when I upgraded from a D60 to the D7000. All sensors above 12mp are more prone to show focus errors in lenses as well as autofocus issues. That is the reason why most of the latest cameras with 12+ mp has a fine tune facility built in. The frustrating part is that it is a sort of an enigma as almost everybody will tell you that you are simply full of sh*t and just cannot take a decent pic :-) The bad news is that the processes being used for testing this and adjusting this all suck in a variety of ways and they are simply not sensitive enough. On all of them you need to make a very difficult assessment with your eyes of exactly what is actually in focus. And after squinting at a number of pics after 20 minutes or so it becomes almost impossible - even for the professionals you are taking the camera/lens to - they do not have anything more special than that available on the web so in the end it is no more than an educated guess. I have found a way around this so if interested give me a shout. I am not sure yet how this site works but I would gladly send you the info on how to do it yourself.