Big Epoxy Granite machine, Which frame is better ?

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I found figures ranging from 12.5 - 28 for epoxy concrete. I'm guessing that the lowest figure is for carefully selected and graded aggregate and the best performing resin, although I can't find out which resin that is.

So, careful mixing would get pretty close as you point out. The next question is what difference would it make ? For example, steel/epoxycrete with a difference of 5um per deg C per metre, with a 10 deg temp change and a 1 metre linear guide means a length difference of 50um, not a lot. I don't know what effect that would have, perhaps someone with some engineering expertise can answer that. In my head I can see the bi-metal strips from school science class, they bent dramatically but the materials were carefully selected to do that.

One thing has become clearer to me - careful mixing is vital - the coefficient of expansion of neat epoxy can be 10 times as high as epoxy concrete, the mix needs to be very even.

The papers I read seemed to say that the major deciding factor was the % of mineral filler, and then the particle size distribution - you seem to need a fair % of small stuff to keep the minimum distance of epoxy small and fill in around any larger lumps. Some people were going as high as 90% mineral, albeit with a material which was little more than sand, hence would pack together tightly - the low % of epoxy would eliminate areas of overly-expanding undiluted epoxy I guess. Removing air bubbles is another issue - good luck with your build!