I own a Wii, too, and the XBOX360 would be the only other system that I would like to have, too. I really like possibility to download demos, addons, user maps, etc. from Microsoft. Nintendo is currently far beyond that and I really don't know why they did so. Is it because the Wii has only Wifi support? Seriously, why not adding a standard network socket to it??

Another point of interest is the XNA argument: everyone is able to create applications and games for the XBOX360. You have to pay a small fee, though, to load it on your XBOX, but it is way better than how it works with Wii-Ware. Actually, it is easier for small teams and companies, but individuals aren't allowed to do so. So, you can only go the homebrew way or develop with a PC middleware engine like GS (when the Wii-mote stuff is ready, though).

I would advise people to buy BOTH systems, if they have the money.