The DDR vs GDDR myth primarily comes from the CAS for the respective memory.
The CAS for GDDR tends to be 2-4x that of DDR. However that is offset by the high frequency that GDDR runs compared to DDR.
As an example Hynix (H5GQ2H24AFR) GDDR5 runs at a CAS of 5-20 (depending on configuration/bus speed). Where as their DDR3 has CAS of 5-11. In all likely hood the Xbox one will be running a CAS around 10 as thats pretty normal at those bus speeds (
http://en.wikipedia.org/wiki/DDR3_SDRAM)
The PS4 memory clock is a bit more than 2x that of the Xbox One. so if we take the worst case CAS of the Hynix memory the latency of both the Xbox and PS4 will be the same at about 10ns.
As far as the DDR5 for CPU issue. I don't think thats a big issue anyways. Anyone that is concerned with CPU performance optimizes their workloads to be Local Cache (L1/L2) bound anyways. The jaguar cores have 2MB of L2 cache. With the higher bandwidth of the GDDR5 vs DDR3 this means that the 2MB cache can be flushed and reloaded 3x faster. So even if latency on the PS4 would be 2x that of the Xbox... it would be a wash.
Now there certainly are workloads that are pure latency bound that can't utilize the L2 cache. But I don't see those workloads being very likely or at least not "maskable" in a system that has an end user latency of 15ms (@60fps).. where as we're talking about latency differences in the 10s of NS.
Now the Xbox One certainly has some advantages, that 32MB memory pool is fast. And if they can fit a workload into eSRAM it can certainly outperform the PS4. Think of a workload where the GPU reads from DDR3 and eSRAM does some processing then feeds that data back to eSRAM for the next phase of precessing. In this case the Xbox One can certainly hit that ~272 GB/sec bandwidth. (IE theoretical max) The question is how much of that workload can actually be achieved?
Also: This is my first GAF post after lurking for many many years. (Been lurking since the 360 launch) so please don't shoot me. Also I preordered BOTH systems.. I'm a gamer and games matter.. and the Xbox One will have some great exclusives. But IMHO it's pretty clear which system I'll be buying most multiplatform titles for this generation.