40K+ Advice on phone exchange

ksud

Contributor
Planning to get a non-pro flagship from one of the Chinese brands.

I have two phones that I can exchange; OnePlus 6T & iPhone SE 2020 (both in almost mint condition). On Flipkart, Oppo Find X8 (512gb) exchange offer is 16.1K for the iPhone and for OP6T is 37% less (10.1K) while Vivo X200 (512GB) is giving an offer of 15.1K for the iPhone and 12.3K for the OP6T (20% less).

While it looks like giving away the iPhone is a better deal, they hold on to their value for longer (plus it's newer than the OP6T) and except for my father no one uses an iPhone at home. We're holding on to it as a backup in case his 12 mini needs some repairs. The OP6T is my current device and while the exchange value is lesser than the SE, getting replacement parts for it will be much cheaper and the screen size way more usable for older folks (they love them big ass fonts).

Which of the two would you consider exchanging? What other factors am I not considering apart from the above? Also why does the exchange value differ from phone to phone (talking about the new phone)? Doesn't seem to be exactly related to the price of the new phone.