I used to use a nice quad core desktop as my main computer, and put an SSD, a hefty graphics adapter, and 8GB of memory, in it to make it perform really well. I needed a desktop replacement laptop for working away from home for a while, and got a nice i7-based, dual disk laptop, and put an SSD in it. Because it's newer, it's even faster than the desktop. My wife's desktop was an older one that performed well, but was using Windows XP and could not be upgraded to Windows 7. So I gave her my quad core desktop, and she's using that in place of the old Windows XP desktop, and is very happy with it.
I've taken the "desktop replacement" laptop and set it up as my new desktop, using a 24" monitor, and wireless mouse and keyboard. It works very well, very fast and nice to use. If I had my choice, I'd rather be using a new desktop, they are quieter, and my laptop just sits in one place anyways. I have an iPad for using around the house, and take that with me on trips. When my laptop dies or becomes unusable due to the increasing demands of newer software (it's great with Windows 8), I'll replace it with a quieter desktop again.