I find that my 18-105 can be soft. I have the Tamron 28-75 2.8 and the difference is highly noticeable. I tested the 17-50 and found it worked nicely on my D90 but wanted the extra reach with the 28-75. The only advantage with keeping the 18-105 is the longer focal length.
Looking at your gear list, I'd go with the trade. Your 55-200 kit lens is much sharper than the 18-105 and you'll still have the focal length while upgrading to a faster and sharper lens.