If my memory is correct, isdn=64k, and isdn2=128k..
Depends how the local telecoms provider tries to sell things.
Either way, there is no difference in cards, there is a difference in software however, not all software automatically uses the 2 channels.
SME uses only one channel, mainly because there does not seem to be enough demand for 2 channels, and the few people that want to use 2 channels can use alternative 'frontends'