When you are a regular employee at an agency you don't have much of a choice on what jobs to take. With an agent you are much more free in that regard.
Being a contractor for an agency is the worst of both worlds: no choice on what to work on that you'd get as a freelance contractor and no career growth wherever you're placed.
I did it for a year and it was miserable. It was great for the first few months, but then I started getting put on more and more menial tasks just to fill out my contract duration and I realized that there was no real point in understanding the systems I was working on in much depth beyond what it took to execute my task.
The other contractors who'd been at it for longer wrote some of the worst code I've seen, even though they were obviously very talented; they just didn't see the point of writing beautiful code if they're not going to be around in a few months to maintain it.
