This same dilemma probably applies to all makes of lenses. After settling for the Canon 60D I am now concentrating on building up my lens collection. However, as a novice I still haven't grasped why for instance would I need an 18-55mm f/3.5-5.6 when I already have an 18-135mm f/3.5-5.6 ? And what am I likely to gain from a 90-300mm lens over the 70-300mm lens I already have? The latter I hasten to add is the much, much cheaper Sigma DG Macro f/1:4-5.6 but it performs fine.
To add to my confusion, I am easily confused as you've no doubt guessed by now why are there several 70-300mm lenses at different prices and quality. Ok, I can hazard a guess that where the letter 'L' is included it has a higher spec glass,they're the ones that usually have me staggering and in need of resuscitation after I see the price I also know that USM will give me Ultra sonic movement when Auto focusing. I understand 'IS' as image stabilisation, but after that I'm at a loss.
Now I am sure there are some really genned up enthusasts on this forum who can help me out on this, so I wait in anticipation for your words of wisdom