I have a digital rebal xti with the kit lense and a nifty 50mm. I've been using this for a little while and now want to step it up a little. I like how fast the 50 is with it's f2.8. I also like the focal range of the kit lense. I don't like how soft and slow the kit lense is. I need a good walk around lense that is fast and has decent focal range. I want to keep it around $500 or less. I have narrowed it down to the Sigma AF 24-70mm f2.8 ex or the Tamron SP AF 17-50mm F/2.8. If anyone has anymore suggestions feel free to add please. Thanks.