The stadium and players names are due to their production dates (and licenses maybe)
WE comes usually just before summer transfers, PES is translated a bit later...
about the gameplay differences, im not sure if there are any, i ve played both 2007 editions but on different consoles (PC and PS2)
Comparing them indirectly with older versions,
it feels like WE, could be a bit "easier" than PES,
but they are built in the same "cuore" and the differences are probable due to the new players ratings only...
A patched PES is the best footballing experiance, but it can be frustrating at times, because of its known issues.