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I'm not sure which two lenses you're talking about. The Canon 70-300 IS does have a USM motor, but it's not quite the "real" thing, as it doesn't allow full-time manual focus, and it's not as fast or silent as the real USM. Not at all bad, though. The other 70-300 is the DO lens, which is one heck of a lot more than $30 more expensive, and most say not as good, image-wise, as the 70-300 IS (non-DO).
What lenses are you looking at? Not the old 75-300 v. the new 70-300? In that case, the new lens is MUCH better, in every regard.
Another issue is that the 70-300 IS is temporarily out of stock almost everywhere, as there was a recall of the early lenses for a fix. Expect to pay ~$550 street price, if/when you find one. I own one, and love it, but it's off for service, and should be home in a few weeks.