Desktops usually, depends on the person though.
If you don't play games or aren't into video editting etc, then desktops are kind of useless except that they are cheaper.
But like william said, overall cheaper, more powerful, easier to upgrade, more customizable. So desktops for me.
Although a laptop for my classes wouldn't be too bad, but those tiny ones cost like...$1400 whereas I built my badass desktop for $800.