WHM vs AST is a can of worms I'm not willing to open.

But WHM and AST is similar to BLM and SMN now:
If you ask "which class is better?", You'll have Pro-BLMs and Pro-SMNs waving pitchforks at each other.
But the truth is, majority of statics/PUGs look for a CASTER, they don't give a shit whether you're a BLM or a SMN, as both of them are fine.

The exact same can be said for WHM and AST, so play whichever healer you like.