I am looking into buying a new lens...I have a 50mm f/1.8...which I love, but I would like something that I can adjust the focal length. Which would you recommend? Canon 28-70 f/2.8 or the Canon 17-55 f/2.8? Also...I would like to save some $$, so is the Tamron lens a good one to get instead of the Canon? Or tokina or sigma? If someone could give me a pro/con list that'd be amazing. Thanks!!