Re: mark 4 vs sightron S3
IMO, You won't find better than a Mk4 -overall.
Go ahead and get the 8-25x, and Alumina flip-ups while you're at it.
I would go Leupold.... Unless it was called 'Leuptron', or 'Leupoldtron'...
Then I'd drop back to the heavier NF NXS.