First, the RAM (68GB/s bandwidth) is not slow. It's design to work with embedded SRAM. This time around the embedded SRAM is more functional all around compare to 360 eDRAM. Nintendo been using this sort of technique since Gamecube. I believe Wii U uses the same technique to be able to overcome its 13GB/s main memory bandwidth.
Certain Dev like Crytek asked for 8GB minimum. The only affordable solution for 8GB of RAM is DDR3. More memory is a good thing. You can have bigger assets, you can pre calculate more stuff and put in that space. Maybe 1.2TF and 8GB RAM + 32MB eSRAM is a nice optimised ratio for given graphic fidelity for MS.
At this point I'm more worried with Orbis only having 4GB, even if that RAM chips are GDDR5. It's like PS3 RSX with eDRAM and not having GDDR3 but relied solely on the 256MB XDR. PS3 Multiplatform titles usually comes out worse in terms of assets, it would look much worse if PS3 only have 256MB XDR, even if it has more bandwidth.
Multiplat Devs will most likely uses the extra performance of Orbis to squeeze in the assets into its smaller memory, you know higher compression, calculate some stuff in real time instead, etc, etc, just to have parity with Durango version.
This is similar to 360 and PS3, but at much worse scale. Sony need to put another 2GB minimum, 4GB would be good. If not they'll just build another expensive console that are perceived as having inferior ports.
Thank you for answering my question ........so we are looking at to completely different disign choices here .
So its logical to think that at the end of the day both machines will perform to there strengths at I right .