Software is indeed missing from the analysis. Now, Sony has two main strengths here, their own franchises like the GT series, and all the best selling 3rd party games like MGS, FF and so on from the previous two generations of the Playstation. It's a safe bet that they'll be able to capitalize on the first with the PS3 as well.
However, developers and publishers of the big 3rd party franchises may have to face a tough decision in the late 2006/ early 2007 timeframe. At this time, if all goes well for Microsoft, there may be 8 to 15 million Xbox360s sold; whereas Sony may still be under 5 million, maybe even 3 million, if they would indeed start in the US and Europe in late 2006. (Note the two "if"s before you start to reply
). These figures are primarily based on manufacturing potential; MS indicated that they should be able to produce 4-5 million Xboxes in 6 months, so their advantage could be anywhere from 5 to 10 million units. Thus, any game released at this time would have as much as twice the market and the sales potential on the Xbox, compared to the PS3. This is very important if you consider nextgen development budgets.
There are of course many other factors that have to be considered here by the publishers. Their costumers may be loyal to the Playstation brand and releasing a FF game on the Xbox could turn into a financial disaster. Betraying Sony in such a way could also have consequences. It's also safe to assume that PS3 will sell very well from mid 2007 and reach a very large user base by 2008.
On the other hand, if the market actually turns out to be that way, then there would be a huge potential for the first (few) big franchises released on the Xbox. We do not yet know all the X360 games in development, but it's a safe bet that most of them are from the same devs and franchises that we know from the first Xbox - so there's certainly room for well-known names. For all we know, these studios might already be watching each others' moves... and who knows how many would follow, if one decides to go for it.
And for all the politics and fan expectations, next gen development is still heavily driven by money. 10-20-50 million dollar projects won't break even with sales under a million copies, and both studios and publishers are driven by businessmen, who probably care more about the math and the balance sheet.
So, in my opinion, software will be the most important factor with this gen as well; but a larger installed base in 2006/2007 may give MS an advantage in that field as well.