iPhone 15 Pro 128GB → 17 Pro 256GB: Exchange or sell ? Upgrade Suggestions

Hi everyone,
I’m planning to upgrade from my iPhone 15 Pro 128GB to iPhone 17 Pro 256GB and need advice on the best way to do it.
Current phone:
iPhone 15 Pro 128GB, Black Titanium
Battery Health: 86%
Used since Jan 2024
1 physical SIM + 1 eSIM
Good condition / no major issues
With the current Freedom Sale exchange offers at Apple, Reliance Digital, Croma, Vijay Sales, etc., is it better to exchange the phone or sell it privately and buy the 17 Pro separately?
For those who recently upgraded:
What exchange value did you get for a 15 Pro 128GB?
What’s a realistic private-sale price for mine?
Which store currently gives the best exchange + bonus/bank offer?
Basically, I want to know the lowest effective upgrade cost. Any recent real-world experience would be appreciated!