I'd go w/ long sleeves if that's what I thought everybody else would be wearing. And I wouldn't roll them up unless I needed to do something that would get my hands dirty. To me, rolled up sleeves look like you're working and are less dressy than short sleeves. Or at least less relaxed, which is what you should be at an outdoor occasion, most of which are more casual than indoor occasions. Rolling up your sleeves makes it looks like you're in the middle of fixing something. I don't think rolling them up makes me significantly cooler, and keeping them down will keep the sun off my skin. Just another perspective.