Well this thread took a rather hillariously entertaining turn, always fun to see "this is how i think this works" presented as factual and used as reasons why X is "better".

Anyway, as always Ghishlain has provided core points backed with actual factual knowledge and experience and thus pretty much addressed the thread point.
never change Healing senpai


As for that last point in the OP, "is one better than the other in some instances", of course; when presented with options, 99% of the time one will always be better than the others for that specific situation at that specific point in time. However for the most part everything is balanced well [enough] and any of the healers are completely safe to use for any given instance, thus that "being better" is a very marginal improvment and you typically will not suffer for not have X during Y.
It's, as always, honestly more down to the player than the actual job/class they're using to perform well and "be better" than other options for whatever scenario your against.
and of course, who or what is better changes through out instances and fights so, you really cant have it perfect unless you literally brought 5 Healers.... which would be a nightmare to co-ordinate and actually make it worse so.....


However one thing i just wanted to tack onto this pros and cons thing is 1/1 Meta points, so

WHM
+You are still widely regarded as the "Power Healer" and will generally be accepted in whatever content you wish, people know what to expect with you.
-You will generally be tasked to main heal fights, possibly even solo for periods, if your not comfortable with this then think elsewhere.

SCH
+You are widely considered mandatory for high end content and will rarely be denied a place in said content.
-You will often be expected to DPS as much as possible and only heal as back-up or to shield Busters etc, if you dont like to DPS, look elsewhere.

Di.AST
+Many now realise you are an equal replacement for WHM and have your own advantages, you shouldnt have much problem finding places for content.
-There is still a stigma surround AST from the past and some people will class you as a "Risk", you may get denied by people who havent moved past that.

N.AST
+Your powerful and have your advantages, and knowledgable people are now recognising that; your not the risk you used to be and some people prefer you.
-You get both the general AST stigma, AND a persistent stigma that your "inferior in all ways" compared to SCH; many will deny you or expect you to go Diurnal.


Either way, you do you and play what you like to, you will always be a better healer for that then going for the "best" healer.