The Cell SPE's are not as general purpose. They're SIMD processors (single-instruction, multiple-data), and don't have transparent access to host memory or the other cores (for some of the Parallella demo's, you can exit the main program and watch the Epiphany cores continue to DMA data straight to the frame-buffer).
They're more similar to a GPU than to the Epiphany. Each SPE is more powerful in terms of Gflops, but the Epiphany CPU's offer more independent instruction streams. If your problem is basically well suited for a GPU (easy to vectorize) chances are it will probably do better on a Cell than the current Epiphany's. If your problem has lots of independent branching, the Epiphany stands a better chance.
