HSM vs. Non-HSM?

[killjon]

I'm looking into lenses, specifically the Sigma 24-70mm
It has an HSM and a "non-hsm" version. HSM is just a faster and more silent autofocus if I am correct?
If this is true, would I really need to spend the extra 300-400 dollars?
I plan on using this lens mainly for video when I get a Canon 7D, and amateur photography as well.
Just want to make sure, and does anyone have any other suggestions for something else?

[ydgmdlu]

Since you can't autofocus while recording on the 7D, you'll have to decide if HSM is worthwhile for your still photography needs.

[johnhafner]

HSM is nice for stills, but the non-hsm is almost as good. For video it doesn't matter

[ryansheffer]

You can't autofocus in the fashion that videographers are used to "continuous autofocus," but you can use the live autofocus function on the camera and hit the "af drive" button and it will find focus. The downside to this is that in order to do so it screws with your image for a split second. If you were doing run and gun documentary stuff it might be helpful, but each time you focus you'd have to be willing to lose about 1 second of footage.

The only reason I bring this up is because the only lens I have used this function with has been my 24-70 Canon L because it has a very fast and silent motor. I can find focus in less than a second. This proved to be very useful when I was shooting video with an underwater housing two weeks ago.