While it is possible that the PS3 will eventually outsell the Xbox360, their real hope is that the PS3 goes one year longer and has that year's sale be better than the Xbox360's last year.

Unfortunately both of these events are getting to be decreasingly likely.  The Xbox was cut short at 4 years due to continual losses on the hardware and the desire to improve marketshare in Japan and Europe.  Game developers did not see the Xbox as a viable platform outside of the US, but saw decent potential for the next Xbox in EMEA and Japan.  They were prepared to make games for the next platform and MS was ready to make some profit so they moved on.

With the Xbox360 we are in a different boat all together.  The system is profitting enough to sustain an entire division at Microsoft that is losing money outside of itself.  Rough estimates put the 360 profits at about $250 to $500 Million per quarter.  MS will not be in any rush to put out the next system with those kinds of figures.

Even if we use your figures of 150k vs 100k per week, we would be looking at 3+ years for the PS3 to overtake the 360.  Now it could be argued that the PS3 might have one more year than the 360 in sales, but even that is looking less and less likely.

Usually the better the system sells the longer it will remain on the market.  The PS2 vs Gamecube is a prime example. (once again the original Xbox suffered from other issues).  Even though the Gamecube launched more than 18 months after the PS2, it went off the market over 3 years before the PS2 has.  It's sales just could not sustain its life.

With the huge losses on the PS3 and the Xbox 360 lead, I do find it realitively hard to believe that the PS3 will have a longer life than the 360 on the market.  It might have the same life, but longer is not likely at this point in time.

To complicate matters further, the 360 has a significant price advantage over the PS3 and will likely have one the entire generation.  Microsoft does not have Blu-Ray or a mandatory HDD in the 360.  As a result, they have initial COGs advantage of about $150 right now and probably around $75 - $100 long term.  The Xbox360 will likely be able to make an even smaller slim SKU than the PS3 due to the lack of HDD and has the further benefits of XboxLive revenue that could help offset costs of a lower priced SKU.

Worst case scenario for Sony is if MS ever decides to sell the 360 like a Cell Phone.  Agree to 4 years of Xbox Live and get a Core for $100.  This would likely be a hit with consumers and only help to drive system sales.  It would also lock in a lot of consumers to Xbox Live and help provide a massive userbase to move forward into the following generation.  Not to mention, it would further increase the userbase of the 360 over the PS3 substantially.

There is a lot that can happen in the next 4 - 6 years.  To assume that one year of games, one price cut, one new SKU release can completely change the current positioning is ludicris.  MS will not go quietly into the night just like Sony won't.  Sony could do a $100 price cut at E3 and MS could counter with even a $50 price cut.  Sony could announce MGS5 exclusive and MS could announce Kingdom Hearts 3 exclusive.  At worst case for MS, they would be about 20 - 50k units a week WorldWide behind Sony after a pricecut for the remainder of the generation.  At best case they could increase to about 150k a week ahead of Sony.  Either way would be a huge shift of about 100k a week, which takes a lot to cause.  Either way, the prospects are not great for Sony winning.

Once again, can it happen.  Yes, but not until really late in the generation.

The bigger story, neither will probably ever take over the Wii.

It doesn't matter if the PS3 starts selling more on average or if the X360 starts selling less on average.  The fact remains the sales difference per week needs to average 77k units for PS3 to overtake X360 in 2 years.

Here is how the math works.  You subtract the PS3 total from the X360 total to get the curent sales gap.   You divide this number by the total number of weeks in question (in this case 104 weeks - 2 years).  This gives you the required average weekly difference that is required.

Again it doesn't matter which console gains or falls, it's the sales difference per week that is important.