At that point I would definitely look at the mileage. You also need to consider the wear/tear on the suspension, transmission, brakes, steering, etc. I've seen far too many people put off regular suspension maintenance leading to other complications later on down the line. If a worn suspension can fuck up motor mounts and tie rod ends, I'd imagine they can rattle those battery packs as well. So I wouldn't think of mileage so much as the "age" of a car so much as it's "experience". Much like how we measure the "age" of aircraft in flight hours. More flight hours = more stress on the frame.

So yeah, more getting to the actual spirit of your inquiry; mileage is gonna matter more to longevity for the vehicle as a whole over charge cycles. On top of that, the battery on the higher mileage vehicle is at higher risk of some form of mechanical failure despite same capacity/lower number of charge cycles.

You should go by warranty remaining on battery. Either one of those could develop a dead cell tomorrow, or they could last 20 more years with no worries. Battery aging is unpredictable on the level of an individual battery pack, you can only predict averages of large fleets. When I used to refurb Macs, I saw failed (swollen or low capacity and about to start swelling soon) batteries with <100 charge cycles and completely fine batteries with 2500 charge count and of course everything in between. 1000 was the expected lifetime per Apple.

I wouldn’t trust the internal diagnostics because car makers have an incentive to lie.

Other components wear with the mileage so I’d look at that.