Depends on the faculty you'll be working with, in my opinion. Some are big on quantity, some are bigger on quality. At this stage, if you're not publishing in JBJS, I've felt that quantity goes a longer way. Pick the attending who publishes the most.
CAVEAT: you'll likely get a letter from the guy you work with, so it also helps if he's a big name, so take that into consideration.
Last thing, pick someplace you want to live for the year (this is all assuming you're talking about a year off for research).