Each one has pretty much the same thing, and they both want the most money they can get. They're in competition. It's like two car makers, they both make car's but they're still rivals because they're in a competitive market.
I'm personally a PS3 guy. I had the first Playstation and have been loyal ever since.
But most of my friends have 360's, so this would be cool.
Though I can see a lot of reasons why it won't/can't happen.