I will suggest to choose from those managers who are currently jobless, he will give good time to your members as he will be only at your campaign, and will satisfy your members, those who managing more than one campaigns usually are unable to support well. You can search on different campaigns, there are a lot of problems to solve, sometimes the manager are unable to reply to all of his/her members are only by saying that 'he is receiving hundreds of messages daily' try to satisfy his members.