Doing a comparison is a little hard at this point because there are so many unknowns about the XGP. Game Park hasn't been very forthcoming about it lately, and as with most things that we can only read about through translated material, there's a lot of misinformation floating around. There's no firm release date for it or even evidence of its being anywhere close to ready for release. Details of its capabilities are sketchy and boastful at best. Whereas the GP2X at least has a (slowly sliding but nonetheless soon) release date, with final specs and pre-release units in developers' hands. There are hard answers about the GP2X, while the XGP (save for a handful of trade show displays) is just one step removed from vaporware at this time, making comparing them pretty much impossible.
If you want my opinion, however.... My gut instinct says that the XGP will be the more powerful handheld but will utlimately flop, either because it's become entirely too ambitious for its developer to actually complete reliably (what the heck is a cell phone keypad doing on there?) or because it seems it's basically throwing away the international market it had with the GP32 by moving away from an open development model.
This is as opposed to the GP2X, which has embraced open development and is providing even better international availability. Though it lacks the 3D accelerator of its half-brother, I see this not as the GP2X being "weaker" but as Game Park Holdings knowing their limits and accepting that a 3D accelerator is a mite too ambitious for a small company. especially one that has to keep costs in check without being sure they'll ship more than several thousand units.
The handheld market is very difficult to compete in, especially outside of Asia. The GP32 is one of the few handhelds that not only didn't drive its creators to bankrupcy but actually made enough of a profit to warrant a successor. (Well, two successors, actually.) This had as much or more to do with its adoption on an international level by the homebrew and emulation scenes as it did with its actual hardware power, and I am concerned Game Park is blind to this fact in their starry-eyed desire to "play with the big boys," not realizing that the big boys play rough and that they are abandoning the fans that made the GP32 a success. While the GP32 may not have been a commercial success in the sense of becoming a market leader, it was a commercial success in the sense that it survived where others had failed and attracted a large and devoted following. A following which has almost exclusively migrated with the original development team to Game Park Holdings' new handheld, the GP2X, which better fits their needs and desires.
And ultimately, a console no one buys will get no games, no matter how powerful it could be in the right hands.