I never got into the shotgun scope thing. All 4 of the Marlin .45-70s I've owned, and also the Cowboy .38-55, were capable of sub MOA accuracy. I didn't downgrade the optics because the guns shot well enough to be worthy of better.
In my SS GG, I started with a Leupold
3-9X which I had handy, then when I needed that for something else, I stuck a 2.5-8X which was also "homeless" at the time on it. Those let me wring out accuracy. I fell into the low power variable trap and put a 1.5-5X on it. I was never very happy with that, the field of view was too great and it was slow to locate the target. Eventually I swapped that for a 1.75-6X.
It's hard to say, for me, which was better, the 1.75-6X or the 2.5-8X. I usually carried the lower powered scope at 2.5 X because that's where I stopped seeing the barrel in the sight picture inside the scope.
All in all, I'd be hard-pressed not to pick a fixed 4X based on past experience with variables .. how I was actually finding them best in field use.