But what about....
There's an awful lot of talk here about hardware's capabilities (not that that's not important, I'm currently multitasking 5 programs on my desktop), but not a lot's been said about the ergonomics.
Where I work, we have to do periodic VDU risk assessments and the safety officer shakes her head sadly at those people risking embolisms, bad backs and RSI when we're using our desktops on a proper desktop. I shudder to think how she would react to the sight of folk peering at a 6inch screen and attempting to do proper work using a squishy bluetooth keyboard, and as for slouching about with a laptop on your lap...
My feeling is that if your tied to a desk in your daily work, get a proper desktop PC. You (and your lower back) know it makes sense...