I made a very conservative assumption, which takes the mindset of the subsciber. I assumed that people want to do file transfers all the time at the maximum rate they could get with the alternative technology. So I assumed that average bandwidth for both cable and ISDN is 120 Kbps. With ISDN, local performance will not vary unless there is some other bottleneck somewhere else on the Internet. But since cable is shared, local performance could vary depending on how many of your neighbors are using it at the same time.
Connectivity is full-time for cable.
If you want Internet over cable, you have to live in Cambridge. Internet over ISDN is more widely available. In terms of limits to growth, the upstream channels are a definite limit on the cable side. On the other hand, ISDN is provided over existing copper pairs. There is a problem if everyone wants ISDN service in addition to their existing analog phone sevice, because there would not be enough pairs.