As others have stated, it really depends on the player and what you enjoy doing the most.

I main AST and mained WHM, but I do not have SCH to 60. This is because I don't like the idea of managing a pet or relying on a pet to help me do my job; I like to be in control of something that could make or break the party. I know, your pet is within your control, but the idea just doesn't sit well with me.

I use WHM for "pure" heals and good DPS output. I feel safe using it because its heals are so potent, so it's some peace of mind when I don't know who my co-healer will be. It also allows me to micro-manage things and switch to Cleric Stance to DPS. With AST, I micro-manage less and am constantly healing/buffing/shuffling through cards/DPS, all in turn.

So if you don't like automatically switching to a DPS when not healing and are okay with trying to manipulate RNG, I would say go AST. If you're okay with only switching between healing and DPSing, go WHM or SCH. WHM is more reactive healing while SCH is more proactive, and I prefer the former. (I also don't like going into Nocturnal Sect as AST because it is considered to be the "SCH" stance.)