No EC is really a MUST. People with all sorts of combinations of ECs can get in. What you have to have is some good ECs, and be able to prove that you know something about being a doctor and want to do it. So shadowing is good, volunteering is good, and you should probably have one or the other. That said, my best friend shadowed a few docs but for only 1 day each, and she had no clinical volunteer experience (but some non-clinical stuff), and she got in everywhere she wanted, including her top school (which is a good school). It totally depends on so many factors, including where you want to be competitive, your GPA and MCAT score, etc. I think research sort of shows dedication, and just looks good because it shows you're scientifically minded or something.
Personally, at this point I'm trying to increase my shadowing experience. I think that will be really interesting for me, and prove that I'm interested in clinical things. Hopefully I can get a LOR from it, too. I am also going to volunteer with the Red Cross at blood drives, but that's really more for wanting to do it than for my application, since I'm applying this summer so it's a bit late to get extra stuff like that in.
Oh, in terms of committment, I really don't know. I think spending more than a week total with a doctor for shadowing would be best. The more volunteer hours the better, of course.
Hope this helps. I'm still looking for advice myself.