Phototransistor-output optocouplers are cheaper and suit for slow digital applications and sometimes for analog applications such as switching power supply feedback. When you calculate the actual switching speed with a reasonably high load resistance (See figure 16) they can be quite slow, but still okay for many applications- such as isolated switches and relay contacts in a PLC. There are also photodarlington optoisolators which are even slower, and a few other types.
Digital output optocouplers are much faster and are specified in terms typical for a logic part, mostly. They use a photodiode on a chip rather than a phototransistor. They also draw power even when off. They are also relatively easy to use (in terms of guaranteeing that they will work under all conditions). You would use them to isolate digital signals, such as for an isolated ADC/front end in a data acquisition system.