What's the difference between these varieties of octocoupler?
This answer (among several others) already answers your question. However, let me add some context.

The basic optoisolator is a light-source (LED) coupled to a photodiode, which is in essence the same thing as a miniature solar cell. You need some circuitry to make use of a photodiode output (or a very large light source and area). These are still commonly used in linear applications to transfer analog variables through isolation barriers.

As circuits evolved, these circuit requirements were incorporated into more usable ICs. Output phototransistors are basically just transistors whose Base-Emitter junction is a photo diode. This provides an amplified output that is easier to use. But a transistor is not very fast driving external circuitry when fast digital signals are required.

This leads to logic-output optocouplers, which incorporate the phototransistor within optimized digital circuitry inside the device to provide much faster digital signal isolation.

The same development path was carried out for different applications. Switching SCRs, switching FETs, switching TRIACs, ensuring that a TRIAC was only switched in the proper phase of the AC line, etc. As more and more applications for optoisolators arise, there is more and more incentive to incorporate that functionality into fewer and fewer optimized components.

This is the normal evolution path for technology in general, and electronics in particular.