I don't personally think the engineering quality of the 58 - Diana's flagship model - is superior to the BSA at all, just that it had extra refinements in terms of the trigger, stock, and loading tap cover.
The prewar Diana models 26, 45, 48 and 58 were all basically close copies of Lincoln Jeffries' original design. You could say the 1930s BSA Breakdown Pattern was a copy of the 1920s buttstock Diana 27 in its turn and the postwar Webley Mk3 was a very clear copy of the prewar Diana 45. So there was a lot of toing and froing of design inspiration between the industrial (and miltary) rivals.
As for the 1950s Airsporter vs the contemporary Diana 50, I certainly see more advanced (ie. economical) steel pressing techniques used on the Diana. But both models were still essentially LJ-type tap loading underlevers and the BSA was probably more refined and elegant from an ownership point of view.