I think both are top-notch units.
The track was a pleasure to listen.
The level and settings differences, as said, make the comparison difficult,
the QES Labs Variable-Gm has much more dBs of compression than the ES-8, that has slower attack settings.
I loved the Variable-Gm sample, for my tastes, it's action and sound fits perfectly in to that rock style,
and the sparkle it adds on the mid-high range is extremely interesting on the final mix.
The relationship between the Variable-Gm and the stereo mix is nothing short of love.
It gives the track that certain something that says, “This is a record”, while with the ES-8 sample, I felt there's still need for further processing.
The feeling I had is that the Variable-Gm sample is some step forward to the final version of the mix.
Even though the Variable-Gm has more compression, I noticed that the dynamic action was very well controlled.
The ES-8 was more on the linear side.
Again for my taste I prefer the sound of the Variable-Gm, with a really shimmery high-end and deep, solid bass.
The high-end on the ES-8 was a tad harsh-sounding, while with the Variable-Gm was particularly nice, as it sparkled but remained really smooth.
It would have been interesting to hear the Variable-Gm with less dB of compression and slower attack settings, I can’t say enough about how hi-fi and clear the compressor sounds when used in these more subtle ways on stereo material.
From the samples of the Variable-Gm I heard, the compressor always added a nice layer of harmonic interest to the sound. I can see why mastering engineers are eating these up.
So my pick would be the QES Labs Variable-Gm.