EPO - heavier but stronger, maybe a little smoother and maybe capable of taking a little more detail but a bit picky about paint.
The detail bit is irrelevant to GWS as they haven't (so far as I know) produced new moulds. My comment about paint adhesion is from my own experience and it dates to kits that were amongst the first produced in EPO by GWS. The latest EPO may be better in this respect, although my experience of EPO or its equivalent from other manufacturers suggests it isn't - paint adhesion is a bigger issue than it was with EPS. Multiplex actually produce a special primer for their "Elapor" which I believe is the Mulktiplex version of EPO.
But basically you're going to have to crash to find out the difference!!