Operationally, there's no difference between the HC-2 and HC-2b *. The difference is in some of the electronics; the older HC-2 charges from a standard DC barrel type jack and requires serial to USB drivers to communicate with the PC while the HC-2b charges from the USB port and doesn't need the drivers. The -2b superseded the older -2 years ago so you wouldn't be able to buy the older one unless a retailer had some very very old stock!
* the software that runs the older -2 may not get as many updates as the software for the -2b.
For the computer interface, I'm not sure if your talking about the CI-1 or the RailPro software so to cover both, technically speaking the CI-1 is a piece of hardware so isn't bound to windows only and that goes for the HC's as well. The software (HC Simulator and RailPro Assistant) yes are Windows programs however both reportedly run well under Parallels and Bootcamp on a Mac, and I've had both running under Wine on Linux.
- Tim