The TechZone.com pulled together a test comparing the performance of WiFi and WiMAX. They discuss the differences for each technology as well as analyzing and comparing their performance. This is a good read if you are entertaining a wireless upgrade and want to see if it really worth the added cost of the lastest and greatest technology.
QOS (Quality of Service) is an major issue with WiMax because of the number of people accessing a tower at once. Some would think that a tower could be easily overloaded with a lot of people accessing it at once. Built into the WiMax standard is an algorithm that when the tower/base station is nearing capacity then it automatically will transfer the user to another WiMax tower or cell. Unlike a Wifi clients who have to kind of fight to stay associated with a given access point; WiMax will only have to perform this hand shake at the MAC level the first time they access the network.