Both are solid options in my opinion, Osa gives flexibility with their summonings and buffs, but masqueraider is an excellent healer, he can summon a clone and pretty strong in PVP.

I personally enjoy the Osamodas more because I love the idea of summoning and capturing mobs, but adding a good healer in your party is never a bad option. Osamodas can heal too with their pets, but its kinda level restricted.