I respectfully disagree with Andy. I think that a research experience where you can leave with something tangible to show your hard work (in this case, a publication) will prove more valuable in the end, especially if you are looking into competitive residencies or fellowships. Perhaps just as important, it can help you narrow down your career choices, as it may influence you one way or the other on academic medicine.
I'm sure that the volunteer experience abroad will probably be a better time overall, and provide you with a few more stories to tell your buddies at the bar, but research will probably be more useful in your career progression.