Either time 10 or 12 Saturday is good for me
The nice thing about inside the South Lobby is that it's air conditioned. But, in the past I don't remember them allowing cosplay gatherings there? Maybe they've set aside an area for this year?
Other wise, outside the South Lobby is good because it's very visible / easy to find. I'm not sure where the other area is, and I know some of the gatherings are in a pretty hidden area (not good for a small gathering: people who run a little late will have a hard time finding it).
So that's my preference... either inside South Lobby or right outside of it.