Dmg orb is usually the best choice as far as ult charge and fights outcomes are concerned. Healing orb is only for when you REALLY have to heal some burst damage or for when your healing juice is low/empty.
Please don't help your team during team fights is what I read. Through one of those out to weaken the enemy, heal with my beam, toss out a heal orb when recharged, rinse, ult, repeat.
Use your fucking heal orb when you team fight, the fuck is wrong with y'all ^ this is why some people shouldn't play Moira. DPS Moira's are hilariously dumb. Heal orb out during team fights, heal with spray as the fight continues. Sheesh.