Newer, better, zoom lens for around $600?
You failed to mention what kind of zoom "range" you are looking for. Wide angle range?, medium range like 24-105mm?, or (usually expensive) medium-long range such as 70-200mm?
Based on the lens you sold and the one you are selling, I'm wondering if it's the 'range' or 'image quality' as your reasons for selling. If it's the 'range' (too long/telephoto), then you probably want something on the wider side (lower focal lengths). If it's the image quality, then check out Canon offerings as well as those from makers such as Sigma or Tamron. New zoom lenses around your $600 figure limit your choices to 'kit lens' quality and speed (f-stop), although there are some surprisingly good values out there such as the Canon 17-55 f2.8 that performs unusually well for its price.
First decide what kind of zoom range you are looking for, and then check out
ebay, Craigslist, etc. You never know what might turn up in the used market at an attractive price. Also, use the 'search' feature on this website (and others) to see what is being said about any lens you are considering.