Orthopedist or podiatrist for metatarsal surgery? Why?

Ask those who know. Framing the question as you did, asking in an either or fashion, makes it impossible to answer. You are painting the canvas with too broad a brush. If all surgeons were equally talented, then it would make no difference as to who you had do your surgery. If you believe that, then I have a gold mine in Indiana I'd like to sell you. Best to ask those who assist surgeons who they would rather go to.
BOARD CERTIFIED. As long as the orthopedist is a foot/ankle specialist, or the podiatrist is board certified by ABFAS.org, either is ok.
Either. Assuming the orthopedist specializes in foot....both are well qualified...