Any Quantaray is over-priced poop. Don't do it.
I have the 28-90mm and I'd be happy to give it away if I could found someone who'd come take it. I'm afraid to touch it as it's problems might be contagious. (OK, so I'm an L slob, but this lens gets straight "poor" across the board at www.photozone.de
and a final score of 0.69, which is the absolute lowest score in the "Standard Zooms" category.)
The Sigma rates "average, 2.5" on PhotoZone so this isn't bad for an inexpensive lens.
The Tamron rates a 1.14, which isn't surprising for a 7x zoom - even though it's tempting to have that wide a range.
From a quality perspective the Sigma 24-70mm seems like a decent lens for the money, and it's a very nice range for "messing" around with - but be prepared for it to be too short sometimes, and not long enough other times. But it's a good range for a lot of things.
Or you might want to think 50mm for a while. The Canon 50mm f/1.8 can't be beat for $70. Very very good bargain for the money, very good quality, and the wide aperture makes a lot of low-light situations possible.