Well, I see three key elements:
1) 360 has a more narrow set of owners demographically, and it has grown on back on Xbox as the console for online shooters/MP in the US. This means that the 360 can shift very high numbers of shooters to a relatively small install base. It also means that there is less risk for MS as a great shooter will always tend to do very well on the console.
2) PS3 has a broader set of owners demographically, and has a relatively weak install base in US, which is the territory which really embraced console shooters. With only relatively few of the former PS2 franchises actually out on the system, and with others going multi-platform, the console simply isn't set up to show massive numbers right now. The PS3 needs a bigger install base and more diverse library to shine, as a price point drop to boot.
3) 360 one year lead and price advantage has enabled the 360 to remain in the better position for key titles, with Sony marketing seeming confused since the console launched. This coupled with the US install base and a strong move in UK and some other territories to 360 vs PC gaming puts the 360 in the better position to post bigger initial sales and bigger overall sales LTD.
Right now I'd say good PS3 titles show better legs, while 360 has better initial sales and has a core set of titles that will always show really big numbers. I'd be willing to bet that the core 360 audience is willing to spend more than the average PS3 owner and is more fanatic about certain 'must have' titles.
Blu-Ray has nothing to do with it, apart from its impact on PS3 price of course IMHO.
Try to be reasonable... its easier than you think...
