To answer the OP: I wouldn't buy a new 2nd phone just to spare battery degradation on your main phone. Not a good investment.
- Replacing the battery over time is smarter.
- If you still want a backup phone that you don't use at all normally, just get a used (or refurbished) iPhone
- If you really want a 13 mini, get one either way! If you fall in love with the size class, there may be a chance that you end up completely switching over and selling your 15 Pro
Yes, it was strange to read this! The 13 mini is much better in many ways, the bigger battery in the 13 Mini is a major very noticeable improvement over the 12 mini. It's a waste to let it sit in a drawer and not the other way around.
If I were you I would instantly start using the 13 mini and keep the 12 mini as a backup in case your main phone breaks, just to cover the time to get a replacement 13 mini.
It's mostly an aesthetic choice as I vastly prefer the jade(?) green of 12mini , it's used caseless with just a magsafe wallet so doesn't show finger prints/oily streaks like the darker green of the 13mini. Also, I'm not a heavy user so the 12 does everything for me and then some.Its interesting that you use your 12 mini as your daily, and your 13 mini is your spare/back up device. With me its the other way round!