Basically, the biggest difficulty of porting a game to any console is just that, the fact that it is a console, no fancy smancy MMU on your processor, no virtual memory, no hd etc etc. The advantage the xbox has compared to a ps2 (cannot compare 360 to ps3 due to lack of ps3 devkit overhere ) is that it shares parts of its api with the win32/dx api and that the ps2 doesnt have an api worth mentioning. And then again, if you really want to use max performance of your xbox you ditch that shared part and use the platform-specific instructions which give you more power ofcourse. (If i am not mistaken someone once made a gfx lib for the ps2 too which was similar to OpenGL, so porting an openGL game to ps2 would have been easier too, same story). If we suppose we have a game on pc which can run theoretically on ps2 specs (rendering and so on) the initial port to xbox will be quicker to do than to ps2 (especially for developers with 0 console experience), but to get the product on that console in a finished state the benefit in the long run is not that big. The biggest issues to solve when going to a console is the list mentioned above, and that goes for any console, be it an xbox360 or a nintendo DS. What i mean is that the step from pc to ANY console is big, if you have no experience in the domain.
Of course developing for xbox is easier and less frustrating than for ps2 if you want to max out the performance, due to the heaps of tools microsoft gives their developers (if you want real-time advanced profiling on a ps2 devkit, you need to buy some piece of hyper-expensive hardware which literally reads out signals on the circuit-board and tells you where the stalls happen , its what they used to get the max out of GT4 on ps2, always handy when you have some Sony engineers to explain you what happens dough ... ) but the market has dictated otherwise and there arent that many games that really push the xbox (due to ps2 being the big player in that generation, so very few xbox only titles with all the dumbing down associated with it).
Btw, porting to the Nintendo GameCube was also not so hard. I never heard anyone say thats "just a pc with fixed specs in a box".
a lot of blabla on Xmas day, i hope somebody is interested in all these ramblings