Nikon 18-200 VR or Tamron 18-270mm VC ?

I currently use a D90 with the Nikon 18-105mm F3.5-5.6 VR lens, as well as a handful of primes.

Now considering getting a "superzoom" to take away on holidays so I only need to carry 1 lens instead of multiple lenses. I have 2 questions please, would appreciate everyone's comments:

1) Nikon 18-200mm F3.5-5.6 VRII or Tamron 18-270mm F3.5-6.3 VC? Each has its pros and cons according to various reviews & comparisons. Is the Nikon worth the considerable higher price?

2) Is there any point in keeping the 18-105mm if I end up getting either the Nikon 18-200mm or the Tamron 18-270mm?

Just to clarify - I'm not after the ultimate in optical quality with this "superzoom" lens - I will use my fast primes for that purpose. It's purely for travelling.