These days, the answer is laptop more often than not. Apple’s 15″ MacBook Pros have similar speed, storage, and more versatility than their desktop computers. Both use similar Intel Core i7 quad-proccessors, and have the same amount of RAM. The MacBook Pros come with solid state drives, which also contributes to noticeably greater speed.
Sitting at your desktop, if you connect a large external display, and a USB or wireless keyboard and mouse to your 15″ MacBook Pro, you’d have all the comfort and speed of a desktop computer…but when you leave, you could also just take the MacBook Pro with you. Try that with an iMac!
It’s harder to make the same case for the 13″ MacBook Pros or MacBook Airs. Unlike the 15″ models, the 13″ models use Intel Core i5 dual processors…dual instead of quad, meaning they can achieve roughly half the processing speed. You can still connect an external monitor, keyboard, and mouse, but a recent model iMac would be much faster.
Companies that don’t want their employees to take their computers (with company data) off premises would do well with desktop computers, and because demand for desktops is way down, they’ll save money…prices for desktop computers are cheaper than for equivalently powerful laptops.
If you want the fastest computer you can get for the least cost, a desktop is the way to go…realizing, of course, that they’re not portable.
For the rest of us a laptop is a better choice at this point in time.