Which agency would you like to represent you?
Some agencies will "represent" anyone as long as they own a camera and have some idea how to use it. Since some (many?) photographers work on spec, it costs the agency nothing to represent anyone and everyone.
This is the definition of hamsterization.
I've been to events where there are half a dozen photographers, or more, all claiming to be working for Zuma, or Getty, or Wire Image, or Corbis, or Wenn, etc. (i.e. 6 or 7 photographers from Zuma, 6 or 7 photographers from Corbis, 6 or 7 from Getty, etc).
"Agencies" these days do not represent anyone except themselves. They are stock picture stores, not true agencies. Although, I stand to be corrected since I suspect some smaller agencies may still operate as true agencies.
What kind of work do you shoot? Which online picture store would be best at licensing your photos? Bigger is not necessarily better.