The clock speeds won't differ much, and GDDR vs DDR is swings and roundabouts. Both consoles have basically the same graphics architecture, PS4 has used the excess transistors to introduce more shader cores, Xbox has used them to introduce a local cache on the DDR RAM. The memory architectures on both consoles are broadly similar, DDR3 has faster overall access times whereas GDDR5 has a much higher throughput at the expense of latency.
I'd expect the Xbox to have better memory performance, both in latency and access to the 256MB (or whatever it is) of commonly used stuff. The PS4 has 50% more computation to throw at graphics. I think the PS4 will be overall faster, especially as developers get to code to the underlying architecture and not via Hyper-V.