The other thing to keep in mind is the cost. Your epoxy grout will cost you at least 5 times as much as a good sanded grout. This is the main reason that we tend not to use epoxy unless it is specifically called for. Unless it is for a specific application (mostly commercial) people simply don't want to pay for it. If cost is an issue, you may be better to lean towards a good sanded grout with an additive or a sealer to be applied after the grout is cured.

Height is point where we are concerned, but if I choose 1/4" Durack we would have to probably put mortar underneath.Than height compare to 1/2" Durack would be the same.

You have to place thin set mortar under all backer boards, then fasten. The thin set will add about 1/16" or a bit more.