What brand of 18-125 or 18-200 zoom should I buy

Hi Guys and Guyesses. This is my first post. I have been looking at buying a K100 but now I'm looking harder at the K10 Because of the added dust/seal issues and of course the greater pixel numbers and optional grip for portrate oriented shots. I anticipate purchasing an 18 - 200 'tourist zoom 'initially to use for a month long Central European work/holiday before buying wideangle and prime lenses for my work back here in Australia. My question to the forum is; 'What brand of 18-125 or 18-200 consumer zoom should I buy'? Ís there a lot of difference between for instance Tamron and Sigma brands'? and should I be looking at a different brand ? Thanks in advance for your considered help.