After experiencing some recent Slightly Dodgy Potential Client Behaviour, I thought I should say a little something about what an author should do if he or she finds that he or she has two or more agents or publishers to choose from (and if, in the latter case, there is no agent to advise).
We'll vault past the initial querying stage to the point where your full manuscript has been requested by an agent or publisher (editor, if you're in the northern hemisphere). Let's call this agent Agent A. If another agent, Agent B, then requests the manuscript, the polite - and professional - thing to do is to email Agent A and let her or him know, so that Agent A is aware that there may come a point at which the author tells Agent A that he/she has been offered representation by someone else; plus it may cause Agent A to hurry up with the reading. It is also polite to let Agent B know that Agent A already has the manuscript, so Agent B, too, can hurry up with the reading.
If Agent B reads faster than Agent A - which can happen for various reasons - and comes back more quickly to the author with an offer of representation, the polite and professional thing for the author to do is to contact Agent A and say something like, 'Another agent has made an offer of representation, but I'd still really like to hear from you. It would be great if you could let me know what you think of the manuscript by X date [allow a week or so].' The incorrect thing to do is to contact Agent A and say, 'Another agent has offered to represent me - see ya.'
This is the incorrect thing for the following reasons:
1. No matter how excited you may be about Agent B's offer of representation, how do you know if Agent B is the better agent for you when you haven't given Agent A the opportunity to tell you what s/he thinks?
2. It's kinda rude, and publishing people value manners. Gods help you if you ever decide to leave Agent B and seek out Agent A again, because Agent A will remember you and will likely say 'no way'.
Agent B will not withdraw the offer of representation if you stall for a few days - not even if you say why you're stalling, because Agent B would expect the same if s/he was in Agent A's position.
In short: it costs nothing and takes very little time to be courteous to people whom you hope will support your career for years to come. This also applies to dealing with people you meet along the way in publishing - today's editorial assistant is tomorrow's agent or publisher. Why not behave in a professional manner towards people you expect to behave professionally towards you?