My own feelings :
Cacko : same as the original but (a lot?) better. The only one which communicates with outlook.
OpenZaurus(OZ)/opie: qtopia like. 100% open, fast. Use more modern gcc libc versions etc than cacko. Not 100% compatible with qtopia apps.
OZ/gpe: X11 base, not mature enough imho though it works and has some nice features.
OZ uses the openembedded build system which is used for other devices as well (openwrt, I think dream also want to use this one)
PdaXrom: if you want X11 only, perhaps not the best choice if you want a pda - by contrast with a mini-laptop/ desktop linux. No qtopia apps run on pdaxrom.
Try them all!