The difference in resale for iPhone 12 Pro 128GB and 256GB on sites such as Swappa, eBay is only about $25-$35 at the moment and thats usually how it is every year so its negligible. If you do a carrier trade in they dont care if it is 64GB or 512GB so you will get the same value. I dont think this should be the deciding factor just get the size that works for you.