The only difference in the hooks is that one is an older hook and one is a newer hook. I don’t know which is which, neither is documented on this site, so it’s hard to say. They are called one right after the other, so it shouldn’t matter which one you use.
I did some testing and I think that you are running afoul of WP’s meta box order setting. When someone edits a post the order the meta boxes are in are saved so that the next time that user edits another post of the same post type the boxes are in the same place. In my test I could not effect the position of existing groups in any way, but I could change the position of a new group that had never been used before.
Go into the db, in the usermeta table and find the meta key “meta-box-order_event” for your user ID and delete it. Then load an event page and see what happens.
