It will be a darn site quieter when running than the 360 once you get XBMC running. It has built in software to slow down the cooling fans to reduce noise so I can hardly hear it. I actually set it up for my mum, who got into using it quicker than the Sky+ box.
Plus if you upgrade the HDD (easier than you might think), you can copy across all your music, pictures and video (via FTP), and won't need to stream from another source (saving electricity).