GDDR favours bandwidth over latency, while DDR doesn't make that compromise.
Though I wonder if there is any drawback for having GDDR5 for all the memory. As far as I understand, GDDR was designed with graphics in mind, but I don't know how different it is with normal "general usage" DDR.
GPUs are able to hide the latency, which is why gddr it works so well for them.
I suspect one of the reasons they made that choice (besides simplicity, i.e. "the radeon already uses that tech and we want UMA") is that ps3 developers are already used to branchless code and dealing with LHS (load hit store).
I wonder how OoOE, branch prediction & high latency memory will mix though. May be they'll strip the OoOEs and branch predictors, since they're expensive and power hungry.
Edited by Matias Goldberg, 21 February 2013 - 02:22 PM.