i've played with many canon USM lenses to know that they are super fast at focusing and are silent while doing it. does anyone here have experience with both Canon USM and Sigmas HSM? im a bit strapped for cash and the sigma 70-200 2.8 is rather tempting looking vs the 70-200 f4 canon. i just wanna know if the focusing speed is comparable between hsm and usm. plz no crap about how sigma sucks and canon is better.