In general, the higher level the mob the better, so under normal circumstances you'd want to kill things in SB zones. This temporary boost only applies to BLUs though so just kill whatever mobs you'd be killing anyway.
Also don't forget about feeding. With a clean stable each feeding is worth 1% of a level up to rank 10, and .75% from 11-20. It may seem small, but it adds up over time, especially as your chocobo gets higher rank and .75% of 3 million is still a lot of exp in one go.
