Both, but as far as photo editing goes my desktop is what I use. I kind of have clearly defined roles for each system. My desktop is actually currently lower powered than my laptop, I got a new laptop after the whole dual core thing went mainstream and it's tremendously more powerful than my single core desktop. This is a cycle that tends to repeat with me, I upgrade each out of phase with the other, so that one is always newer than the other.
But my desktop has all my editing software, it's my supremely customized system with all the special little programs like encoders, editors, codecs, plugins, and so on. Also it's got the monitor that I trust, my laptop display seems to tend to blow out any highlight areas on photos even if they should look okay. Maybe I could calibrate it, but.. my tendency is to use my desktop for photo editing anyway, so I leave the laptop as it is rather than risk compromising the overall appearance of the display in the name of forcing it to match a defined standard. What I mean is that LCDs only have so many bits of dynamic range, when you calibrate them you're constricting the dynamic range of certain color channels to get them to match a certain color temperature.
My laptop is a portable gaming machine, portable web browser, AND, most importantly of all for me, a portable writing center. I write fiction as a hobby, and find that the more relaxed setting of sitting somewhere with a laptop on my lap is more conducive to the creative process.