Volunteering gives you more transferable skills and says more about you as a person than shadowing does. Long term volunteering positions - ideally with different client groups ie say, elderly people, children, or people with disabilities - get you the chance to show responsibility and to develop skills working with people over a long period of time and give you an idea of whether you will actually enjoy working with people. By comparison, shadowing basically says you can fill in forms, send some emails, and can walk or sit down for however long you happened to do it for.
Yes, I’m being slightly facetious. But the ratio really shouldn’t be tipped towards work experience. Work experience should give you an opportunity to identify skills and qualities necessary to do the job and then relate it to your own skill set and personal qualities that you have developed in various settings and roles. And one set of work experience is pretty much the same as another once it comes time to you actually condensing all of this into a personal statement. And work experience/shadowing will count for almost nothing in the interview, in which questions tend to be formulated around “tell me about a time when you…”
The Manchester form for medicine is actually really telling. People who have done little else besides shadowing tend to have almost nothing to say on that form and really struggle to pad it out. If you look at that form, you’ll get an idea of the sorts of things you should be able to talk about in a personal statement. Likewise, look at interview questions for medicine and try to have a think about what sort of answers you would want to give.