You're probably OK in either case. I'm going to guess that your best bet in terms of performance is actually the ATI chip with the closed-source drivers. That's for two reasons: the ATI chip is actually superior hardware, and Intel's Linux drivers, though open-source, tend to be considerably slower than their Windows drivers, while the ATI blob has comparable speeds on both platforms.
If you want an open-source solution, the Intel works *now*, while the ATI open-source drivers won't be up for high-performance gaming for quite a while yet (even the supported cards don't have all of OpenGL 1.4 yet, and they're several times slower than the blob for 3D.) This won't be fixed until all of the ATI cards are supported (the priority is to bring 2d and basic 3d acceleration support to *all* cards, then move the 3d up to more advanced gaming-suitable level of support and performance.)