Private shops are great if you can stick with them and do good work... the opportunity for advancement is much greater. At the same time, you could get put out of business easily. You never know until you've worked at the shop for a while which direction its headed.
Best buy/circuit city is a better bet for most people, you'll get paid more to do the simple stuff then you would at a privately owned shop. Now, if you can fancy fiberglass and things like that, a privately owned place is the way to go, just a matter of finding the right one.