Do both if at all possible; lab positions are usually 9-5 or so, so there shouldn't be any reason why you can't volunteer on the weekends or in the evenings.
I would like, however, to emphasize something that has disconcerted me from the first moment I read the original post: choose something that you are EXCITED ABOUT and INTERESTED IN, not something that you think will look "good" on your application! The question that you've posed to us -- should I do this or that, which would be better for my app? -- is really irrelevant, because the sort of activities you're describing should be pursued out of an interest within you. If you run through the motions of selecting an experience to cross it off the medical school app checklist, the results will be counterproductive: medical schools can tell the difference between people who become involved b/c they think they're supposed to, and those who do so because they're passionate about a subject. And believe me, they prefer the former.
On that note, allow your own interests to guide you when making your decision. Choose an activity that you could conceive of continuing beyond a few months. I'll give you an example. . . I've volunteered in the pediatric department at Memorial Sloan-Kettering Cancer Center for two years, b/c I absolutely love the people and the children there, and in fact I applied to medical schools in New York, in some part, so that I could stay and continue to volunteer during medical school! During the summers, however, we see a lot of students who come and volunteer for a three month stint. . . they're not very engaged with the children, and then they always leave in the fall, obviously not bringing very much away from the experience. For them, volunteering at Memorial was just a matter of "checking it off the list," and they didn't select it based upon their own passions. Another activity -- one that possibly they could have realized if they had put more thought into their actions -- would probably have been better suited to such volunteers. It's pretty apparent when they leave that they haven't taken much away from it.
So please, let your own passions be your guide!
Best of luck to you in your decision. .. have a wonderful summer!