B has a husband man. You have emotions involved and you're not thinking clearly. Where do you think things will go with B? Best case scenario she'll break things off with her husband to be with you, but do you really want to be with a girl who cheated on her husband? She'll do that to you too.
I think you should go with A. It's not uncommon for relationships to end and then reform. When they reform they are better than they were the first time because both parties are more willing to put the effort in to changing to what their partner prefers. The reason they break up in the first place is usually because of lack of emotional connection, lack of communication, or suffering in the bedroom. You and A have most certainly discussed these problems before and if you are able to get back with her, you have to make it clear for each other why it failed the first time and that you want to prevent that from happening again. So you discuss the problems and don't repeat them.
Don't continue things with B. You're being a scumbag by helping her cheat. She really should get a divorce at this point.
Cheers, brother
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