Which Lens? -- Nikon or Tamron

[Its-Jake]

I have a Nikon D90 w/18-105mm lens that I really enjoy using. My only regret was that I did not buy the 18-200mm lens at the time I made my purchase back in December.

I am now considering upgrading to the wider range lens but since I lost my job in April the price of the Nikon AF-S DX NIKKOR 18-200MM F/3.5-5.6G ED VR II has now become a bit out of reach. After doing some research I found the Tamron AF18-270mm F/3.5-6.3 Di II VC which is considerably cheaper and something I can reach for.

My concern and question to the group is will I lose anything in performance and quality if I go with the Tamron vs Nikon? While I am not a purest I would love to keep a Nikon lens on the D90 but have to consider my current financial situation.