They're really quite different, so I have a hard time making a comparison. They might as well be in a different class, in my opinion.
Anyway, with the Octave Multiplexer, you can get one of the most versatile monophonic
octave sounds available, because you have seperate filters for both your dry tone and your octave, along with a blend knob to adjust the level between the two.
The POG, on the other hand, is more like the EHX Micro Synth because they both synthesizes multiple octave tones blended in with your dry sound, then passes all of them through one filter.
For filter junkies and those looking for just a plain old octave sound, the Octave Multiplexor might be an appealing choice. You can take the balls out of your octave tone (which sounds really cool), and at the same time fatten up your dry tone. On the other hand, if you're looking at an octave pedal for making your guitar sound like a synthesizer, go with the POG, or better yet go with the Micro Synth.
I suppose the next thing I should explain is the difference between the POG and the Micro Synth. The biggest differences are the POG is polyphonic, can generate tones an octave higher than your dry tone, and has a more simplistic filter system. The Micro Synth has probably the best
filtration system on any pedal, synth or otherwise, because you get resonance control. I'll let you pick which sounds more appealing.
Sorry to bury you with information.