Honestly I think volunteering and shadowing are weighted the same, however with volunteering I feel as though you get more hands on experience. Shadowing is just when you follow around a pharmacist, see what they do day to day, and take notes . I feel as though it may be helpful to do that once or twice, just to see directly what it's like in the day of a pharmacist whether it be retail or hospital. I volunteer in a hospital pharmacy right now and they allowed me to shadow a pharmacist once. However during my volunteer hours I'm allowed to batch, package, put away stuff, and follow the techs to their respective floors. So i'm allowed to do supervised tech work and be involved in the daily runs etc. I would recommend volunteering over shadowing a pharmacist every day, BUT if you can actually get a job being a pharm tech, do that instead, you get all the experience AND you get paid and you don't have to be as supervised.
I volunteer one day a week for 4-5 hours because that is all my school schedule allows, do as much as you possibly can though.