Personally I like to have both. My desktop is for watching movies, buring DVDs, playing games, and watching videos through my TV. My desktop has 5.1 Surround sound and all my consoles and DVD player run through it for 5.1
At the same time, my laptop is great for the internet. I can use it in bed, and I can whip it out on the fly to place a bid on E-Bay or to fix a problem on RPGN because I have mobile broadband. I have WiFi at home so they both use the internet so I can be downloading on the desktop and browsing forums on the laptop.
If I had to choose one, it would be the laptop for portability.