There are too many technical differences to mention them all. A GPRS cellular system has paging channels, pilot signal channels, and extremely robust handoff triggers, etc.

The most basic difference between GPRS and WiMax from a user perspective is that the handoffs in a GPRS system work seamlessly so that there is no interruption to a phone call during a handoff (and even if there is, the connection will automatically recover). WiMax does not have the same robustness so a key test will be whether it will be possible to reliably maintain a VoIP call on a Wimax system when traveling by train or car from one end of the city to the other. If there is even a split second interruption, the VoIP call will be dropped, it will not automatically reconnect, and you will have to redial.