congrats!!! to me the tank size is ok right now for 1.5", but definately will need bigger tank, 'cause they eat alot and grow quick with water change. IMO i would go BB for now, cause you're feeding them BH, BW... those food can stay under gravel, you have to do water change at least every single day to make sure clean/good water for 1.5" discus. but don't worry, if you feed them good with clean water, they grow quick.
Good luck
Good luck