Desktops are really nice to work with particularly if you invest in a larger monitor with high resolution. But laptops and netbooks are portable. As a compromise, I use whatever laptop I currently have to run a normal mouse, keyboard, and monitor at home. It lacks the power of a high end desktop, but you have improved ergonomics and productivity.
Currently using a Macbook Pro 15" with boot camped Windows 7. It is versatile, reliable, and reasonably powerful. But it does cost a premium for the hardware specs. I have it connected to a dell 30" monitor. Thinking of getting a larger harddrive for a larger Windows partition and speed (7200 rpm hybrid). I will have to see which OS is in use at graduate school. Also considering bumping RAM to 8 GB. Less of a need and more like a want though.
I use Linux, Mac OS X, and Windows. So although I really like the mac, I am not tied to all things apple. Almost any laptop can serve as the brains for a hybrid desktop with addition of monitor, keyboard, and mouse. And you don't need to buy it all at once. An option to consider at least.