As I understand it, it's more the case of which kernel for which machine.
The 760/860 with Sharp ROM (and thus Cacko) came with 2.4.18.
The 6000 had a variant of the 2.4.18 kernel as that model used the Toshiba "companion" chip for video and SD/MMC. That Tosh chip is mostly undocumented (presumably because SD was a secret standard at the time) and remains the stumbling block to getting accelerated video.
The x000 series came from Sharp with 2.4.20.
Older builds of OZ (and a relatively recent one for 6000) used 2.4 but all the recent releases use 2.6
I think stable versions of pdaXrom use 2.4.18 or 2.4.20 (much as Cacko does), but some versions in testing for some hardware use 2.6, but I don't know to be sure. Note that pdaXrom and OZ builds are incompatible because one uses hard float, the other soft float; I can't remember which is which.
Angstrom uses 2.6 (being, I suppose in some ways a fork of OZ) but uses EABI, which makes it completely software incompatible with anything else, but is a much better fit with the Arm architecture and is faster - by how much I don't know!
Please do correct me. Perhaps this should be put in the wiki once all amendments have been done.
-- edit --
found a wiki page had already been started:
https://www.oesf.org/index.php?title=Kernels