I’ve never had a Loquelic or an AT, but I went from HD mk2 to ER-301 to HD mk3.
I’m still undecided about whether I prefer the mk2 or mk3, and would have to try them side by side I think.
The mk2’s unmodulated sines are dirty, aliased and characterful without being harsh. Its FM sounds really good, and it’s very immediate and easy to use, one knob per function etc. It’s got a sine/triangle/saw switch which also affects FM, and a square output too. The “waveform discontinuity” shaper is pretty bad, though.
The mk3’s unmodulated sines are super clean. The waveshaping is much better. Having one more modulation oscillator and the hybrid mod matrix/bus gives it superpowers – it can imitate the mk2’s noisier core or get more in Noise Engineering territory. You can also use operator A as a second carrier, which can sound pretty great in stereo; you can separate its pitch control from the primary and use it independently.
The mk3 can get much busier/crazier than the mk2 if you want, but both of them are also good at gentle and pretty. In their brighter ranges, they make excellent LPG food or percussive sources.
There are times when something tells me there’s something about mk3’s FM character I don’t like as much as the mk2’s, but it could be nostalgia lying to me, so I’d have to try them side by side. I’ve definitely hit some mind-blowing sweet spots with the mk3 though.