Preferred,
Here's how I look at it. Granted that if your building your first machine from scratch, a store bought PC will be cheaper (the big companies get their parts much cheaper than we can). But, once the machine is built, there are many advantages.
1. You're no longer afraid of the infernal machine and whats inside. Once you've built one, your generally no longer afraid to do just about anything inside.
2. When something does go wrong, you have a pretty good idea of how to troubleshoot the problem and replace parts if necessary.
3. In the long term, it is much cheaper to just swap out parts than it is to replace a whole machine every two to four years.
Personally, I'd never buy another desktop or allow a friend to. Once you have the knowledge of how easy these things are to build, it really makes no sense to buy a major brand. Besides, you also don't get a machine filled with spyware right from the factory.