Given the commanders you mentioned, if you are thinking of going into MLD tactics I would say that Kaalia is the way to go. The advantage with Kaalia is that she gets into play quickly allowing you to seal people off with the MLD. The other two are a lot more situational and Avacyn takes a lot of time to set up making a board wipe / MLD following her up kind of predictable.
I like Bruna and think she does angel tribal decently I think she is better if you aren't going to be playing with or against MLD and competitive combo decks. Ultimately I don't think that mono white is a great option if you plan to play against opponents who are going all out in commander. Kaalia probably has a better chance if that is the kind of environment you are expecting but I also think it might be worth your time to go in and watch a game sometime to get a better feel before jumping to any conclusions.
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I have officially moved to MTGNexus. I just wanted to let people know as my response time to salvation decks being bumped is very hit or miss.
I like Bruna and think she does angel tribal decently I think she is better if you aren't going to be playing with or against MLD and competitive combo decks. Ultimately I don't think that mono white is a great option if you plan to play against opponents who are going all out in commander. Kaalia probably has a better chance if that is the kind of environment you are expecting but I also think it might be worth your time to go in and watch a game sometime to get a better feel before jumping to any conclusions.
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[Modern] Allies