Uh...none of the above...

Vince,
1) Tamron is generally thought to have better quality control than
Sigma, on this level lens you will either get a marginal lens or a
stinker. With both companies you should buy the lens from a local
company so you can test the one you are going to buy.
2) Trying to get all you lens range in one lens is a mistake, you will
get much better lenses if you split the range between two lenses.
Tamron has a 28-80 Di XR (maybe 28-85) lens that has had rave reviews.
Then getting a 100-300 pure tele zoom lens. This combination would
improve the picture quality greatly. It is much more fufilling to get
sharp images.
A tip on reviews http://www.fredmiranda.com/reviews/

Tom