Disclaimer: I don't work for the Xbox group, I just write games. We didn't even get to find out the real name before the MTV show...
When Sony came out with it's comparison slides at E3 early this week it seemed that the PS3 had much greater performance, but when you start thinking about the one cell chip on the PS3 versus three PowerPC chips (with two hardware threads, plus all the one dot product per cycle stuff), the numbers really start to look suspect.
I'd planned on sitting down and doing the math myself to come up with some real numbers, but Major Nelson over in Xbox land has already collated some real comparison data in a four part series over on his blog.
Now we have sensible numbers. But there's another angle. Writing multi-threaded code is hard. It's especially hard when the components you're targeting don't look the same. The PS3 reminds me somewhat of the Sega Saturn - ask any game developer who has been around for a while what writing code for that was like...
Simulations (read: games) are going parallel which is hard, but worthwhile. But it's a lot easier if you're running on hardware threads with the same architecture - and you'll effectively have six of them on Xbox 360 (three CPUs with two hardware threads each).
But try doing that when you have one CPU and seven DSPs to offload the math.
Not to mention the memory bandwidth. Memory bandwidth is often missed when consumers or the press try to compare platforms. The fact is that the speed of the game (and therefore the amount of cool visuals and audio you can present to the end user) is gated by the amount of data you can move around on the system. Especially between the CPUs and the GPU and the GPU to the frame buffer, not the mention the Hi Def frame buffer.
From a software development standpoint this generation is going to be an interesting one as the simulations go more parallel. Typically it takes a few generations of titles for a particular platform for developers to figure out how to eek out maximum performance and find all the tricks they can play. In order to accelerate that process, it comes down to tools, education and developer support. For that, the Xbox guys win hands-down. Xbox and now the Xbox 360 proves that Microsoft knows how to put the best tools and developer support in the hands of the developer.
From my software developer centric point of view, the XBox 360 is a more symmetric, balanced and capable platform.
But don't just take my word for it. Go and read the Major's comparison.