Before I compare and contrast the two, I'll give you a brief overview of each technology. This service offers one of the fastest connections that a home user can reasonably afford. It is a true 64k or 128k connection. By true meaning that the upload speed and the download speed are the same. This type of connection is also a true digital connection.
Regular modems take a signal and convert it back and forth between analog and digital.
Because the signal remains digital the whole time, it requires special wiring that the Phone Company must provide. They not only have to string the wire to your house, it has to be wired right up to the jack in the house that you will connect your ISDN modem to. Remember though that normal modems are limited to a 33.6k upload speed but with a Shotgun setup that too can be doubled. So now that the background information has been covered, now to the meat of the subject. Like most things in life, it is a trade-off.