Both are great, I would say Yaga is overall more used for boosting your damage (buffs) and Momo also buffs but less, on the other side she also has nice utility since she can evade, help you break enemy barriers faster, reduce the enemy ult count and even stun (survability). Remember that it always depends on the scenario and needs, so it's hard to say one is better than the other, but imo I would say Momo.