whms throughput cd's divine seal/presence of mind + there 3 off gcd big heals is what makes whm a better choice and people know that.

As someone else said, it's not about potencies, it's about hard healing cooldowns that matter, divine seal regens, presence of mind cure spams, full heal off gcd tetras, insant no mana aoe assizes, swift cast holys, the list literally goes on.

And this is why there is a thousand threads on the issue.