It's semantics. There are classes that take more getting used to. Such as a disc priest or hpally's cd juggling, or the insticts required to get the most out of druid's hots. On the other end, there are more user-friendly healers like shamans or holy priests.(easy to pick up, hard to master.)
I'm not a huge fan of the last sentence. I don't find discs, rdruids or even hpal to be non-user friendly. If anything mistweaver ain't too bad either. The only complexity in the class appears once you have to jabweave juggle over say strictly meleeweaving or casterweaving. With that said though meleeweaving is very easy to pickup, just quite suboptimal and casterweaving also is user friendly.
I mean at the moment, bad discs can still do great not even pushing their spirit shell button. An almost-optimal rapture can be done simply by PW:S a tank on CD (assuming you are not on tank duty because then you would reduce the debuff duration) but let's be honest, PoH, PoM, Cascade and SS are so strong a RL would be crazy to put a disc on tank duty.
In a similar way, bad hpals can do completely fine not using any of their healing cooldown (they still need to use divine plea obv). The class is not by any mean any harder than rsham from a user-friendly perspective.
What I'm trying to say is, as it's been said already, this is such a subjective debate. I agree that "complexity" can be a mean to make the debate somewhat more objective...O wait complexity highly varies from an individual to another so not really. To some players, "complexity" when healing comes down to "when to use what cooldown" making hpals and discs sightly harder from that perspective. To others, complexity comes from having a tighter rotation in which you must use a given spell (or a few given spells) on CD (like the ReM discussion about monks...or riptide on shamans...holy shock). Then again, most classes. Furthermore there are some players that find complexity in juggling friendly+enemy targetting juggle putting priests and MW as hardest.
Anyway I've played every healer class although definitively at a different progression level (few mostly as raid pug alts) over the years. I'm currently playing MW main and after the nerf I rushed my disc up in case we'd need a solid disc for progression. I definitely don't find either to be any harder than proper execution of riptide and unleash+healing rain which is what generally seperates good rshams from bad ones.
As to help the OP chose his next healer class to level, here are my advices, suggestions.
To a classical "healers only heal", mistweaver will appear to be the hardest to pickup from scratch and it would help your main raid if you have a MW cohealer as you would then know their mechanics :P.
If you wish to gear at a superfast pace (for a strong alt run or good pugs etc), take Hpal. The 4pc pvp bonus is so strong you can leverage the gearing process doing a little bit of pvp and pve. Learning to optimize their countless amount of cooldowns is also quite refreshing whilst not really forcing you to change your playstyle too much.
If you just wish to %@@@*%#% LFR meters. Go disc. Preemptive healing > all. It is also a wise choice as a potential future main even should they nerf them simply because absorbs have always provided means around specific encounters mechanics making discs "encounter breaking" on HM progression.
If you want to put up to a challenge, try competing with similarily geared/skilled friends while playing a Rdruid alt :(. You could also just pick that class because by the time yours will be geared for content, 5.2 will be out and your class should have changed quite a bit!