The single ring is a cleaner look when you carry the bag without a strap. Theoretically the double ring is better for the bag if you usually use a strap, especially if you carry heavy things.
I wouldn't decide based on rings but on strap - if the less expensive bag doesn't have a strap and you need one, getting an Hermès strap to match will take a long time and cost a fair amount of money.
If you don't intend to use a strap with your Kelly, by all means, get the single-ring bag. Certainly no one will care (or notice) if your bag was made before 2000 or later. A Kelly in good shape can last forever (I have one from 1952).
I agree with the essence of this.
Nonetheless, I think an important consideration should be which bag is in the best condition.
You should also try to work out if you would want to use a shoulder strap frequently.
Kellys were used with a shoulder strap for many years before the introduction of the double ring.
With a single ring there might be wear to one side of the leather where it is attached to the bag.
With the double ring the hook does not touch the leather and the weight of the bag is now better balanced.