This is a two-parter. So, first:
If you receive an offer of representation from multiple agents, is it wise to tell the "losing" agents why they, well, lost?
I'd take a two-pronged approach.
First, sort your reason into the following categories (if you have other reasons, let me know in the comments section and I'll sort them for you):
- Editorial vision--that is, the other agent had a vision closer to what you were thinking.
- You just had a feeling when you heard from the "winning" agent and/or there was a feeling of fate (weird coincidences/things in common). Hey, it happens.
- The losing agent's sales record/years of experience.
- The losing agent's contacts/experience in your genre--perhaps he/she said that this was a new area for him/her, so all of the editors who'd receive your work had never heard from this agent before.
- That agent's other clients--perhaps the "winning" agent has a great big list of bestsellers.
- You don't like them. Maybe they sound neurotic on the phone, talk too fast for you to hear, or seem untrustworthy.
- You're worried that agent won't make time for you. You seem to be a low priority.
- The agent just seems mean. You're afraid he/she will yell if you say the wrong thing.
- You were otherwise offended by said agent.
- You think the agent is too old/young/unpleasant/poorly read/evil.