i've done both. presently i'm working for myself.
it pretty much boils down to your personality and what you want to do with your self. big firms take away a lot of hassles of equipment, clearers, money. But equally so add hassles of dealing wih people, restrictions on time, and processes.
Working for yourself allows more freedom, more money in your own pocket if you are sucessful, more stress. For me, i would not go back to working for a big firm unless i really screwed up. but thats just my personality.