One consideration (this is a generalization, but most generalizations are based on some facts) is that usually the longer the zoom range, the quality of the image will suffer. As an example, the mega-zooms (28-300mm for example) will usually result in softer images (especially at either end of the zoom range) than a lens with a narrower zoom range (such as 24-70mm).
In any case, look for real-world reviews and photo samples before making a purchase.
I really like the lens review section of FredMiranda.com for getting real-world reviews.