http://www.microsoft.com/xna/faq.aspx
Unfortunately, it's like choosing the lessor of two evils, SCEI standard or MS standard, as neither of them are open...Q: What's the difference between Microsoft's XNA and Sony's Cell?
A: In the next generation, software-not hardware-will drive the games industry forward. Microsoft XNA software brings together chip and OEM partners to ultimately deliver thousands of integrated digital entertainment devices that work together and give consumers choice. Sony is talking about a fixed world of hardware that requires everyone to buy everything Sony. Sony's Cell is a hardware solution. This is a software revolution.