The main advantage of the GPE rom at the moment is that it runs on X and uses gtk2 - This allows you to run the vast majority of desktop applications without modification, which is always useful (Think Firefox and Thunderbird as your browser and e-mail client). It also looks much nicer than Opie, although that's just personal opinion.
The disadvantage of GPE is that the core applications aren't nearly as developed as the Opie ones at the moment, so their stability/features aren't that great. If the PIM functions mean a lot to you, I'd say go with Opie - Software support (at the moment at least) is also much greater on Opie. However, if you like the potential to run most Linux applications and the power that's afforded to you by running an X server and having the gtk+2 library, go with GPE.
There's also the case that out of the two images (not counting bootstrap-image of course), GPE is the only one at the moment with working media-players (gpe-nmf for audio, or totem for audio/video). opie-mediaplayer1 segfaults on startup and opie-mediaplayer2 segfaults as soon as you try to play anything - These errors will be fixed in due time of course, but for now, this is the case.