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Which is better?

Comments
Sym Supreme is more niche and matchup specific but he is a monster in the fights he works for.
bwcv is better probably as she can do more matchups since she only needs immunities on champs (bleed, poison and incinerate)
Sym supreme needs buffs a bit more than bwcv
personally sym supreme is my fav
But I can tell you if you are planning to do story content sym supreme will make it easier in act6
6.2.6 champion boss is too easy for sym supreme (yes, people will tell you bwcv can do it too and heal too during that and all that stuff) but its much easier with sym supreme
and 6.3.1 medusa has a "do you bleed node?" with buffet and bleed vulnerability
Sym supreme is the best option for that. He makes that fight super easy.
BWCV is one of the few poison immune mystics which you want to handle v5 scarlet witch boss and she will probably be a bit more useful in EQs and stuff
You would wanna do both but depends on what you wanna do first