As a WHM main who recently levelled my AST for the hell of it, yes, it is very similar when utilizing one of their two 'stances' (not correct term but similar principle), seeing as one essentially has you play just like a WHM (healing-over-time) whilst the other is more about pre-emptive shielding (Noct). Some may appreciate it more given that cards give you something to manage during downtime or on the fly, whereas WHM is far more straight-forward... Either case can be viewed as positive/negative depending on the player.

Ultimately, I still prefer WHM and its simplicity, especially as a DPS minded Healer. There's very little between my primary responsibility (keeping people alive) and the ability to 'contribute' something else - no cards or fancy mechanics to worry over, just "Is the team healthy? Yes - Aero/Stones. No - Do they need to be healthy? Yes - Heal. No - Aero/Stones. Meanwhile AST can do the same, yet there are (slightly) more MP concerns, especially levelling when you will almost always need to hold the odd Ewer due to scaling/damage and/or inexperienced players and/or going ham on damage because you can.

If I were you I'd personally see a Healer through to the end first and then consider dedicating spare time to the rest. The 'slump', as it were, only really applies to the upper-crust - and even then it won't be mentioned over-much other than from a balance perspective on the forums or by groups who are inclined to cherry-pick (which there is no major need to at the moment). It's not like WoW where they've butchered, say, Prot Warrior so hard that it sees about 5% participation due to being obnoxiously bad compared to almost any other option. Yes, WHM is behind the others, but not to such an extent that it'll effect most people.

I had fun levelling my AST and it now gets to enjoy my WHM's 380+ set, but when I really want to sit back and enjoy my role, I'll always deviate to good ole WHM.