Yes, nobody can heal effectively in cleric stance. SCH and SMN at endgame also have different equipment sets. One gives INT while the other gives MND. You will literally have 300 points of difference between the two stats if you are appropriately geared, so your SCH will never have very high INT. Cleric stance also reduces your healing power by 35% despite your lower INT being used for healing. Simply put, you will output more healing if you are NOT in cleric stance no matter what stats you have.

I would go 15/15 for the time being. You can then play with 30INT or 30MND, depending on which class you end up using more. As you gear up though, you will find that SCH depend less on MND and more on Crit (you lose 40hp on adlo and 50hp on physick with 30INT compared to 30MND, which is not a big difference when your tank has 9k hp), while SMN depends quite a bit on INT because of the way the dots scale up.