Where Parallella's advantage comes in, is its grid architecture. Whereas a typical GPU today is "very very wide" (IE: an AMD 7750 does 512 operations at the same time), the Epiphany is a "grid", and each node of the grid can be doing different things.
I'm going to greatly simplify things down for you with this example (so experts out there... don't shoot me :-p). A GPU can execute 1 program, but that one program can do 512 operations at once.
On the other hand, the Epiphany-IV truly can run 64-different programs at once.... all taking different paths and doing their own things.
Epiphany-III and Epiphany IV perform "if statements" more or less how you'd expect a computer would do it. Which is the important bit... if-statements don't really slow down your program.
In contrast, the typical wide "wavefront GPU architecture"... the "if statement" basically halves the speed of your GPU. The GPU has to execute one "half" of the branch, and then later, it comes back to execute the other "half". (Its only "really" executing one program, but doing 512 of them at once. See what I mean?)
GPUs are a "wide" architecture... but Epiphany is the first "grid-based" supercomputer for ~$99. GPUs are very mature technologies... Epiphany is still in initial research (as far as the consumer market is concerned)
EDIT: In reality, a modern GPU supports maybe 4 to 8 different wavefronts (8 for the more expensive GPUs, maybe only 4 for the cheaper ~$99 GPUs). But each of those wavefronts can do hundreds of computations at once.