If the Transmax is the Dex/Merc type sold at WM, of the two I'd pick ML Full Synthetic. Transmax D/M is mineral based ATF, still ok however my preference for the syn and it's more shear resistant properties. Castrol Transmax Full Synthetic at AAP equivalent to ML, wouldn't pay more for it than ML, unless I suppose qt bottles preferred.
As noted, Dex 3 not a tough spec to meet now and superseded long ago.