Question: One or many Behavior Trees?

My question is a simple one (I hope unless there is more under the hood than I understand):

When I program an AI I have certain things that an NPC can do and they are all exclusive, so either they do A or B but not a mixture of both.

Should I create multiple behavior trees, one for activity A and once for B and then decide when to run each and tell the AI controller to switch, or should I run Tree A and then stop it and activate Tree B when necessary?

Thanks for any advice