If that's what you're intersted in, you need to run Opie, period.
GPE actualy runs a variation of the X windows System that Linux on the desktop.
This is nice in the sense that it's pretty, and can run some more powerful apps if you put the work into it. However, it takes a lot more CPU power to crank it over. Also, at the moment, the GPE version is VERY broken IMHO.
So, if you want a super cool AND super geeky interface with lot's of potential but you're also willing to put in lot's of work, GPE is cool.
If you want a basicaly finished and pollished interface with lot's of apps that a monkey can install to make a useful and user friendly tool at the same time, use the Opie interface.