Is there any possibility that you can do both? Medical schools do like to see research experience, not necessarily lab work. . . although it is also true that clinical exposure and volunteering is key. Is there any particular reason that you have to choose one or the other?
Most importantly. . . which would you ENJOY more? This should be your main criterion on which to base your decision. Not only will you be much happier if you choose activities about which you're passionate, but that excitement and dedication will shine through in your application. What types of experiences are you considering? Which interest you the most?
Very best of luck to you. . . whatever you decide, have fun!!!