If anything thinks the xbox arch is more advanced than the ps2...they're smoking crack.
However, the r5900 (ps2 main processor) isn't 128-bit. It's 64-bit, conforms to MIPS-III, and partly to MIPS-IV.
The bus is indeed 128-bit, but that doesn't make the chip itself 128-bit. The two vector units are 32-bit with a 128-bit bus. The IOP is your psx processor that does all input/output related functions. The only problem is the arch is a little too complex to many developers. Sony didn't do a great job on their docs for it, so having to learn all the VPU/GIF/IPU/SIF on your own made development hell. Thats where I think the xbox has one up on the ps2, the XDK being mostly about examples and includes a framework. I could go on about the archs for awhile, but it is mostly pointless.
Also, anyone who thinks of the p3/p4 itself as a CISC chip need to learn about modern processor design. Only the ISA is cisc. All instructions get converted to micro-ops, and from there on it is hard to tell what type of chip it would be.
If it was an r12k in the ps2, the story might be a little different about which is better, since the r5900 is only 2-way superscalar.
From the technical standpoint, I like the ps2 arch more, and I cannot wait to see how they design the ps3. From the practical standpoint, it is far simpler to crank out code for the xbox. Too bad there aren't any great rpgs out for it yet...
(And to anyone who thinks FPS are where it's at, why are you playing on an xbox, your selection is so few, and not as much competition compared to playing on the computer.)