The X360 is a multi-core system based on PowerPC much like the PPE on the PS3. Having taken a look at the chip of a RRODed console myself I can safely say is physically impossible to the Xenon process more than 75.2 GFLOPS/sec (6 threads, 3 cores, 8 single precision floating points per clock cycle no double precision, 1 MB of shared cache). 
In the other hand, PS3 has the PPE with 512 KB of cache, 2 threads, and 7 SPEs with 1 thread and 256 KB cache each. All of them process the same 8 FLOPS per clock cycle and the SPE can still do 64-bit double precision. This equals to 204.8 GFLOPS of single precision floating points per second on the PS3, more than twice the processing power on the X360. Look below how the PPE looks exactly the same as the X360 CPUs.
That's the main reason PS3s are clustered and not X360s arcades. Even costing half of the price PS3 is still a better choice. Indeed, the Cell is the best cost-benefit processor on the market. Based on the same architecture, both consoles must have also the same % of theoretical max used.
Of course for gaming there is still may factors going around, like front bus, RAM amount/bandwidth and GPU power. Going by gaming alone, X360 has the best value out on the market.
