I think ps3 and 360 will continue to sell virtually the same amount globally for the rest of the cycle. I don't care what people say, the ps3 and 360 are virtually identical systems. Sure they have differences, one is more powerful, and the other has a better online experience. As far as entertainment goes they are extremely similar. Gears or Uncharted? Fable or GoW? Dumb questions, all 4 of those games are fun.
There are a few types of gamers out there, that can purchase items that is. ( I am about to paint broad assumptions of human beings, you are warned ;)
Teenagers
Parents with kids
The guy who lives with his girlfriend/fiance
Single people.
If you are a teen, you can probably only afford either the 360 or ps3 and it would be difficult to spend the cash on games for both systems. (ps3/xbox)
Parents with kids want a system that they can play a few games they used to love, and games their kids can safely play. (wii)
Girlfriend/Fiance/Married dude: Lets face it, I got the wii and wii fit to appease my girlfriend before I bought a 360. Granted I love the virtual console and a the ninty exclusives. I buy nothing else for it. (wii)
Single People: You guys rock, buy whatever you want and how ever much you can afford. (xbox/360/wii)
I know thats broad, but it makes sense to me about the numbers.