Durock comes in a 5/16" thickness, too. That is a hair over the 1/4" thickness of Hardibacker.
I think Durock is almost all concrete sandwiched between 2 sheets of mesh. Hardi has some kind of fiber mixed in it(I think they call it "post consumer material").
I have used both, but the last few times I have tiled I went with Durock.
I am sure either one would be fine.
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