Analog with great tracking - I've had good experience with the OC2 and Bass Octave Deluxe, however for analog *beat the f!%k out of your speaker* octaving with great tracking, my Ampeg Sub-Blaster slays all.
I believe the OC3 is digital but without any of the advantages of the pog/hog series ie: 'tracks' whatever you feed it. and the POGs will track whatever you want, but sometimes it does sound a bit fake.
I'm currently rocking the POGII (8 presets, 2 octaves up and down, attack delay, chorus - I use it for alot of different sounds), Ampeg Sub Blaster (for *traditional* octaving) and an OC2 (for synth-bass sounds)