If PS3 sells 12 million per year it will take just over 2 1/4 years to reach 60 million. In the same period of time 360 only needs to sell at a rate of 9.5 million per year. Easily beatable by 360. 12 million per year by PS3 is quite likely over the next couple of years.
If PS3 sells 13 million per year it will take just over 2 years to get to 60 million. In the same period of time 360 needs to sell at a rate of 10.3 million per year. Also pretty achievable by 360, in the last 2 years 360 has averaged 10.6 million. At least as achievable as PS3 selling 13 million per year for the same period of time, possibly more achievable.
If PS3 sells 14 million per year to get to 60 million then 360 will need to sell 11.1 million per year. I think it's a tougher ask for PS3 to sell 14 million per year than it is for 360 to sell 11.1 million.
Overall the probabilities are in the favour of 360 getting to 60 million. And I definitely think both machines can be on the market long enough to get there. 70 million would be a very different matter.
Edit: minor correction
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