The main division between Palms has to do with their other port; this is either an RS-232 serial port or a USB port. The USB ports are of course faster (though not as fast as you'd think; there are limits to how fast Palms can drive data), but not as universally supported.
Some models of Palms that use USB ports (notably the Sony line) have proven difficult for Linux to support. Support for these models is being worked on.
(BTW, just because a Palm can hook to a PC using USB does not mean that a Palm can hook up to, say, a USB printer and drive it. USB isn't a peer-to-peer protocol, it is master-slave. There must be a host PC on the bus to manage communications.)