To say Bard/Sorc or Bard/Wiz is supperior to the other you need to make a comparison to judge their usefullness, and as a healer you are supposed to heal, you can aid with stuning during AoEing lured mobs, but thats it.
Bard/Sorc gets: Song of Winds, an EVA envoy group buff (+467 EVA at lvl 48 ) a AoE debuff remover which is also a group 12 second HoT, 9,5 seconds CD with a SPD build.
Bard/Wiz gets: Sonic Shield, a damage reduction envoy group buff (-451 damage reduced at lvl 51 ), single target debuff remover+HoT with a 15 seconds CD with a SPD build.
That is their difference that matters for a healer, to me it comes down to : Do i wa nt the -451 damage buff, or an additional HoT which is also a debuff remover?
I personally went for Bard/Sorc to get the extra HoT which is also a debuff remover so its quite awesome imo, both are viable healers and depending on what boss you are doing one is slightly better than the other, but none of them is required to do a certain boss except for Bard main ofc hehe