Just done some digging into a more detail on the hardware for both consoles. Some of this you may know, some you may not, interesting non the less. Being a programmer myself, I found some of this interesting from a developer perspective.
Both consoles share a very similar Accelerated Processing Unit, which consists of CPU and GPU processors on the same chip.
The CPU's on both consoles APU's are based on the AMD Jaguar line of processors, both will also have 8 cores, reliable rumours say that the XBone's CPU will run at 1.6ghz, I would expect the same for the PS3 CPU also.
What is pretty much confirmed though, is that the XBone's GPU is based on a AMD Radeon HD 7790. While the PS4's is based on the faster AMD Radeon HD 7870.
Both systems also use 8GB of unified RAM, unified meaning that this memory is shared between both the CPU and GPU, which makes sense from a performance perspective given that the central and graphics processors are shared on the same chip.
GDDR3 for XBone which according to developers will deliver 68GB/s bandwith between the CPU, GPU and memory. But the GPU also has a 32MB block of local SRAM with 102GB/s bandwith between itself and this memory.
GDDR5 for PS4 which will apparently deliver 176GB/s bandwith between the systems vital components. It may seem like much more bandwidth but as I mentioned earlier the XBone mostly ameliorates this difference with 32MB of high-speed SRAM on the GPU.
The bottom line is that the PS4 with its faster graphics processor and one single big block of high speed memory should deliver slightly faster performance and slightly better looking graphics, for £100 cheaper too. But they are still very very similar in performance.
No doubt that a modern PC will stomp all over both consoles in terms of raw computational power. The current gen Intel quad core I7 processors are roughly twice as fast as the 8 core AMD Jaguars that are seen in these consoles, in terms of raw computational power. As for graphics cards, the latest graphics cards are absolutely miles ahead of what's going to be in these consoles.