I do use zeiss, leica, swaro, NF, Burris, Vortex
From my experience the Zeiss, Leica and Swaro are far superior in:
- low light (light transmission)
- fine details
- no curvature of field
- more field of view
- Border quality ( no noticeable difference from center)
- no flare (reflection, fuzziness typically when use with strong light in frame)
As per scientific/metric comparison, There are a lot of comparisons typically on camera lenses.
There again there is no contest Leica and Zeiss are ahead of the others including the high-end Japanese brands (Nikon, Canon, Fuji) Sony use zeiss glass and panasonic Leica glasses.
The difficulty to manufacture that increased quality explains the cost difference: the R&D to achieve additional improvements, the glass type, glass treatment, assembly, assembly process, tolerances, skills of workers, etc.
Is it worth the extra money, up to each of us to decide.
For me yes - for the type of local hunting i do - local regulation allows to shoot very early or late so wining a few minutes might bring a dead red stag.
For Africa, it is up to debate.
For instance, I use a NF 2.5-10x32 on my 458 Lott - glass quality is goo enough and it is small and robust... and cheaper that a German glass. Yes I would prefer to have a Swaro Z8i 0.75-6x20 for the field of view but the price difference meant a that I could shoot an additional plain games on last safari.
If I put a scope on my driven hunt rifle (Browning Bar 9,3x62) i will make sure to use it also when I go on safari but for now I rely on a Leica tempus red dot (also a lot better than a Burris fast fire)