The thing you have to remember is that many Tamron or Sigma lenses are reverse engineered from either Nikon or Canon, and sold as equivalents. This is especially true of the 70-200mm focal range, as that coveted ground is currently flat out owned by Canon. That being said, the two best lenses I've heard about from Tamron and Sigma are...
Tamron's 17-50 NON-VC is sharp and highly recommended, especially for the price. Definitely test it out first, peoples tastes and feel for lenses vary wildly.
The Sigma 8-16 & 10-20mm take great pictures, despite the distortion the naturally exists at such wideness. Every picture I've seen from the former lens typically looks breathtaking, the latter is always paired as its equivalent in quality, but my experience of scouring pictures for the 10-20 is a bit limited.
Sigma also makes a lot of super focal length lenses, but they're so highly priced that I'm not even going to bother getting into them.
That being said, Tamron and Sigma are really a mixed bag. Many of their lenses have many positive aspects (price being a major one usually). They also however have many drawbacks, and typically these con's show up as harder AF tracking, increased CA/Vignetting, and a weaker body structure. Sharpness may or may not suffer, it really depends on the lens.
I only recommend the Canon 70-200mm because it flat out has the best optics available in that focal range. You might want to go to a store and test those lenses out and get a personal feel for them before following my (or anyone's) recommendations. If I were you though, I would just save up for the Canon variant, because it would be cheaper in the long run (you wouldn't have to sell and re-buy gear after all).
Also, invest in a good tripod!