Personally, I'd swing towards the laptop. Change of opinion as I had a laptop years ago and it was rubbish! Have always been a desktop kinda guy, as I like to mess around with them, tweak and upgrade them. Now have a Vaio SZ series laptop, and have never been happier with a PC!
Small, light, battery lasts for 5+hrs when it needs to, screen is a decent size and the processor is pretty good (dual core widger, more powerful than my desktop anyhow!). Have bought a freeview tv tuner that's the size of a usb memory stick, and my laptop is now all I need to take with me to home bravo. Awesome piece of kit and highly recommend it, especially when combined with wireless broadband.
Did I mention it's carbon fibre and weighs less than 2 kilos
Not advertising vaios in particular, but hey! (Bond uses one as well in less you havent seen the shocking ads!)