Mac-vs.-PC style "ad" for There and Second Life.
The comparison to Macs and PCs, though superficial, works fairly well. The two worlds do a lot of the same stuff -- user-generated assets and events, virtual currencies -- but There is prettier, less flexible and customizable, and has less market share. Second Life offers more customization, is more popular, and erm, is ugly. In a number of senses of the word. ;)