Canon 100-300mm f/4.5-5.6 USM AND Canon 90-300mm f/4.5-5.6 - Which one should I get?

[amoki]

Right, I am planning to get a medium telephoto lense. Both of these are imports of course. But there is a difference - the former has USM, the latter just an ordinary zoom lense (with micro-motor, isn't it?)

The two websites:
1)http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=12059&is=GREY
2)http://www.digitalfotoclub.com/sc/product-features.asp?id=964586679&lhalf=bp

What is so great about the Ultrasonic motor that judtify increasing the price for another $100 (and mine you, the second has has an extra 10mm in focus). And is it worth that extra $100?