Personally, I levelled WHM to 60, then got my AST to 60 the week before SB dropped, and have mained AST ever since (though I'll take my RDM on kill-heavy MSQ quests, since some are friendlier to dps and some are friendlier to healers). It's 100% subjective, but I prefer AST massively to WHM. You get a lot of your "quick heals" a lot faster, meaning you can focus more on dps and cards; the really fun toys don't come until later, with things like Earthly Star, but I really enjoy throwing out buffs and I enjoy the RNG aspect since it doesn't actually nerf my heal ability.

In the end, if you're not enjoying yourself, it might be worth taking the time to level a second class, especially since your WHM progress will always be there if you're bored of levelling something else and just want to go back to the MSQ. So far I've found AST and RDM to be my faves, but again, I understand it's all subjective.