Many people have said "PM is much cleaner." Until I hear from someone I trust with extensive experience with both (I only know X), however, this FAQ will take no position.
There was an unsubstantiated rumor that someone was porting an X library to OS/2 (X-Windows, not X-Mode or any other X), I dunno whether this has been substantiated or not.
IBM and a few other manufacturers have built their own X servers for OS/2. [details?]
[Colin Jensen...]
I have some limited experience with X and PM. Raw Xlib is harder to program than PM. However most, if not all, of the X11 Toolkits are *easier* to use than PM. I have experience comparing XView to PM. A friend of mine has experience with Windows versus Xt, and has the same opinion (Windows isn't that far off conceptually from PM).