I have both. I have the 3-20 on my GAP-10 gasser, and the 5-25 on my A.I.
I absolutely love my 3-20. I have considered moving it to my bolt gun. I just can’t justify changing them. I absolutely think the 3-20 is a better option on a hunting rig. You’re correct on using 3X on the bottom. On a walking hunt, you could jump something in heavy cover where a 3X will be better than a 5X. Also, that 5X will be tunneled where the 3X will not be. There’s not an animal I can think of that I would shoot with that type of scope/rifle combo where the 20X wouldn’t be plenty sufficient. If you miss at 20X, you’re just as likely to miss on 25X.
By your own statement, you won’t shoot past 400. If you even said 600, the 20 is PLENTY. Remember, MANY, many elk have been killed with 3-9 and 4-12’s. I’d much prefer the 3X on the bottom.
From a guy that has both: 3-20 and never look back.