PS3 = teh Cell and teh Blue Rai
Xbox360 = teh Unified Shaders and teh HardDrive
For the PS3 we all know that the Cell processor integrates itself with the RSX gpu to produce movie quality graphics once game programers figure out how to fully utilize the SPE's a few years from now. And the Blue Rai will contribute to it by giving the extra storage space that will be required in the future. At the beginning games will come in standard Dual Layer DVDs since Blue Rai disc are about ten dollars more expensive to make plus the extra storege is not yet required for games. The only problem I see now with the system is that the PS3 doesn't include a Hard Drive in the box so you'll have to buy it apart. Not having an HDD standard means that developers won't take advantage of it for their games since not everyone will have one. I suppose you will have to download game patches to your memory card instead to address the online play glitches that are discovered. All in all, if you own an HD-TV (or two) the PS3 sounds like a good investment since you can also watch high definition movies on it.
The Xbox 360 will have the first GPU ever with an Unified Shaders architecture which will take full advantage of Direct X 10. It will also include a Hard Drive wich is a complete necessity for online. The HardDrive allows you to download patches to prevent cheaters from using glitches and etcetera to ruin the game for everyone. Everyone that games online knows how important that is. The HDD will also be used to download game demos and trailers so that you can preview before you buy which is a great thing indeed. Also, every game allows you to play your own mp3 songs during gameplay. You can use voice chat in every game with the included headset mic. You can also do Video Chat with the camera (not included). The Xbox Live 360 program is the main selling point of the system.
Both systems will offer graphics comparable to high end PC's for the next year or two. We will have to wait a few years until programers learn how to fully utilize the muli-core CPU and unified shaders GPU of the Xbox360 to see what it is truly capable of. The same thing can be said about PS3 and the Cell. Developers have come out and said that they have no idea how to program for the SPE's. So they are using it as if it were a common single core CPU. But all that will change in the future.