Quick guide
NBBA is the top league
Then Division 2 (4 leagues in this division)
Then D3 (16 leagues)
Then D4 (64 leagues)
Then D5 (Right now only bots play there)
Each league is unique but D3's are more competitive than D4 for example. The higher division you're in the more money you can get from TV contract and Merchandise. You'll also have to have better players to compete there though.
Moving to a new D4 is not necessarily good or bad... Just different