I never understood the 'WMC is easier to setup' argument. I tried both and I had an easier time setting up XBMC, all I had to do was point it to the media dir, choose Imdb as scraper and let it go, download a bunch of addons from the official repo. I didn't have to install codecs, install additional programs for metadata, pay for premium skins or plugins etc.
XBMC was much nicer to look at, faster and easier. The only reason I'd switch is if I had a tv tuner card and wanted live tv.