I think you both are arguing the same point.
Bevan had lower-risk run chases thanks to the Aussies good attack, and played in better bowling conditions. Hence the higher average but lower SR and few sixes
MSD has had to chase larger scores in better batting conditions, so more sixes, higher SR, but consequently a lower average.
You can't argue who is better, IMO, because Bevan would have had to bat in a different style to MSD. He would need to be correct technically and more compact to keep out the bowling, given the better bowling tracks, the fine exponents of reverse swing at the time, smaller bats and longer boundaries. MSD's game has evolved to be more expansive, more suited to the big hits and run scoring, but less capable of keeping out good bowling.
Basically you'd back Bevan to seal off an Eden-gardens style 150 run chase against Boult, but Dhoni to take you over the rope in a 350 run chase n India against Sri Lanka.