For typical home use, I'd go with a 3/4hp unit. If you intend to grind
lots of tough stuff, eg bones, then I'd get
a 1hp. I just installed a new Insinkerator 3/4hp unit. Got it at
Lowes. Apparently Lowes has some models made
just for them, as I didn't see them anywhere else. One thing I liked
about these were they had nice polished stainless
flanges that go in the sink drain. By comparison, the Sears units had
dull stainless that looks like crap in a nice sink.
As far as sound, it's pretty quiet compared to the old one it replaced.

One think I did learn a little late, was the Insinkerator doesn't have
a removable rubber baffle in the opening. The opening
is also smaller than the old GE unit I replaced. Those two things make
it harder to get your hand inside to retrieve anything.