Keep in mind that those are both ideal arenas. Don't feel worried about promoting if your arena is behind that. I think as long as your arena isn't tiny (under 10-11k) you can usually manage for the first however many seasons if you continuing expanding it in D.III. Milwaukee Lethargy has been very successful in D.III, D.II and the NBBA while having an arena that was under 15k due to having a good PR manager.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs