EF 70-200mm f/4L IS USM or EF 70-200mm f/2.8L USM........if the price were the same, which would you pick? i'd love to hear from someone regarding these 2 lenses. some of the volunteers at arizona highways use the 2.8 with extenders and are very happy. does the 2.8 give enough more to warrant the additional cost? i've heard that it can be too sharp (don't know what that means) thanks for any advice. do any of you use either of these.