Ad Age has an interesting article on some of the similarities and differences between agency copywriters and freelance copywriters, including work environment, pay you can expect, etc. While not exhaustive, I think it definitely contains some nuggets of wisdom for anyone considering one of the two paths.
Since I've done both, I do feel it incumbent upon me to add my own two coppers for the sake of anyone considering trying to become a freelance copywriter. If you're just starting out, you need to get an agency job.
Who Actually Gets the Work
Without professional experience, even if you have internships, it's going to be hard to impossible for you to find decent freelance work without any real-world work experience. Most agencies and companies I've talked to that use freelancers seek someone with a minimum of three years experience and often are more comfortable with people from five to seven years experience, whom they know they can count on. It also helps, because after working at an agency, you'll understand more about how they work and what they need and be better able to serve those needs.
They also don't tell you how hard it is to find clients and work. Almost all freelance work is project-based, so you'll constantly be spending unpaid hours drumming up new business. You might think that the higher rates you can sometimes earn will make freelancing very lucrative, and if you've made enough of a name for yourself, it can be. However, don't forget that something like 30% of the money you make will need to be saved for taxes and that you always need to live below your means in case business hits a lull.
Most of all, if you try freelancing, keep overhead low. Thankfully, freelance copywriting is something that can be done with a bare minimum of equipment, and if you keep your fixed expenses low, you'll be better prepared to weather low-business times, and be more profitable over all. And isn't it cooler to live in an efficiency and have money to travel to Europe than be stuck at home alone in a giant apartment? I guess that's really your call.
If you are going freelance, it always helps to get registered with a talent agency like Aquent or Art Squad (or whichever creative staffing firms your area might have). They usually won't keep you busy constantly, but they can certainly get you a project here and there and have connections you don't. They also free up more of your time to devote to paid work as they find new business for you. Don't rely on them to run your business, but a project here and there always helps.
Feel free to share your thoughts on the matter. After all, who wants to stop learning?