I have worked for both new car dealers and individual shops and they have their benefits and drawbacks. Here is my take on it:
Dealers: big, cold, all about the money and cranking out repairs fast. The money you can potentially make is very nice, but its a little heartless and depending on the dealer, just downright shady. If you're passionate about cars, changing oil on camrys, brake jobs on pickups, and scanning diagnostic codes can sap the passion right out of you. You most likely will leave every day exhausted and have exposed your body to all kinds of chemicals and physical strain you can imagine, but if you're passionate about that kind of work, it can be very rewarding. I liken it to the different fields of medicine; they're all doctors. To me, working in a dealer's service department is much like working the ER. Lots of high-volume, standardized work, but if your passion is saving lives (or seeing someone happy with a fixed car in your case) then it can be very rewarding.
Individual repair shops: These are like Family Practice medicine; a more one-on-one, voluntary approach to things. Of course depending on the shop, it can be just as bad as a dealer, but I'm talking more about the specialized shops like an independent BMW repair shop, or a speed shop. They tend to get the more varied work. An ER at a hospital sees a thousand cuts, burns, and car accidents every day. Get 'em in, sew 'em up, get 'em out. A speed shop might see anything from exhaust to suspension, to forced induction. Each project is a little more research and custom application.
Jiffy Lube and other national chains: Forgive me, but I have to bash these guys. As a very serious and passionate car guy myself, I wish to rid the world of this stuff. Working at a Jiffy Lube requires that you are 18, you watch a video, take a dumb little quiz on the video, then go start shadowing someone in the shop who has also watched the video. I had used them a handful of times because I drive cross-country a few times a year and I didn't have the means to do it myself when the interval was up. I have horrific stories about those Qwiky lube places that I won't get into here, but I've watched them keep putting engine oil in because they were checking the transmission dipstick, try to find a replacement for my K&N air filter, top off my windshield washer fluid with anti freeze... the list is horrific and endless. They have ruined one rear axle of mine, put 20w50 oil in my wife's Tercel (good thing I was watching) and countless other mistakes. If mechanics and other automotive repair people are the doctors, Kwiky Lube employees are like the night Janitor at the hospital. They might vicariously pick up some little tidbit of actual knowledge about cars, but they are NOT repair technicians.
The other option is some kind of custom rod or restoration shop. These guys are the cosmetic surgeons of the automotive world. They're tough to get in, but if you do well in school, get a few projects under your belt and make a portfolio like an artist, you have a good shot. That's what I did and Hollywood Hot Rods offered me a job... which I didn't take
Stupid, stupid, stupid
These are strong words that I'm about to say, and I'm sure there are many exceptions to this rule, but Qwicky lube places are staffed entirely by shop class flunkies who think that cars are "bitchin" and they need a summer job. I have never seen an intelligent person working at any of them. None of them know a lick about cars. I actually had one guy when he was putting my 96 Impala SS in the computer for an oil change go over to the car, count the spark plug wires on one side, walk back to the computer and enter 4-cylinder. Mind you THIS WAS THE SHOP MANAGER.