Maybe it's because I work in an ad agency as a web devoloper/designer. Or because I love photography, but I have never been happy with the Tamron lens. I have never used Sigma, but from what I have heard they are not much better.
I have found Tamron lens to be more blurry and a compromise. Sure you get a 17-200 Lens for a few hundred bucks, but you get what you pay for. They are blurry and have bad pin cushioning. Plus they are slower with auto-focusing as well as other draw backs. In my opinion any "one lens does all" will be a compromise. it does not matter who makes it. You are better off with 2 or 3 lens to do the job. You will get better quality in the end.
That's why we bought a Land Cruiser right? You wanted a solid truck.
Same with lens you want a solid lens and you will have to pay.
That's why use Nikon ED lens. The cost more, but you get more.
To me Tamron, is like GM or Ford. It's cheap and spec wise it compares with Toyota. But we all know you can't compare Ford with Toyota.
Same goes with lenses. You can't compare Tamron to genuine Nikon or Canon lenses. Most Ford and GM owners are happy because they don't know there is anything better. May I suggest the same for Tamron lens owners.
My advise: Whether you buy a Nikon or Canon, it does not matter so much. As I already said, pro's and con's to each. But don't cheap out on the lens. Get Canon or Nikon lens. There are some after-market lens that ARE quality, but you don't want to know what they cost.
You get what you pay for, never forget that.
I would rather buy a used quality lens, than new crap.
You can try and find a used one here:
http://www.photogon.com
Cheers,
Nick.
P.S. I don't want to sound like a know it all here. But this is my experience.