USM is equivalent to Nikon AF-S, and Sigma HSM. What's available will depend on what body you're going to be shooting.
Thanks to being a new design, the current exchange rates, and generally being the top of the line, the Nikon 24-70 is pretty expensive. Of course it is the best midrange zoom on the planet right now. The 28-70 can be had used for much much less, but if you're going to be shooting on the DX format mainly there are some other solutions.
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