Probably depends on your schedule. With as few little good series there are to watch right now, it's not a bad time to catch up, I would say. Then again, the summer season doesn't look too great, either (other than Hidamari Sketch season 2).
Since the OVA episode is being billed as "alternate universe," there's no need to wait for it. Afterstory, OTOH, probably won't make much sense if you haven't finished season one.