Insight...I think the simple users as you say, should like both really depending on what there is to do. So for the visitor it is more about the content and experience than how it is all made.  SL has the current advantage of variety and scale of randomness. But UGC is often UGG (=user generated garbage). So my net perspective is content is the key and follow the content creators not the platform makers. In fact if you want something simple Xbox, Home, WoW is also simple and social. Seems like you actuallly might want something more than simple IMHO.