While desktops and notebooks are both PCs, the features of each are very different, and for many people will be the deciding factor.
The most important feature of a desktop computer is its versatility. Desktops can be configured with various monitors, keyboards, mice, and sound systems, and for this reason tend to have better equipment and all of these areas. Generally speaking it is more pleasurable to write an email, listen to music, or watch a movie on a desktop.
Notebooks can also hook up to these accessories, but doing so detracts from the greatest feature of a notebook - mobility. While most desktops are used with bigger monitors and keyboards than what a notebook can provide, you can't take your desktop with you on vacation or down to the local coffee shop. If you want to do that, you'll need a notebook. Period.
The question you need to ask yourself, however, is how often you would take advantage of the mobility of a notebook. When I run into someone who was unhappy about their choice of a notebook, it is usually because they over-estimated how often they'd use its mobility. If you like to check Facebook every half hour or you're addicted to web surfing you'll probably use the mobility. But if you only check your email once a day and use your computer mostly for business or to play games you may not use the mobility of a notebook often.