iPhone XR upgrade or battery replacement ?

Yes, a battery below 80% Battery Health will deliver significantly less performance. A battery replacement in my opinion is worth it. If you have money to burn, go for the iPhone 16. I personally would choose the former.

What important new feature or features.....which you need..... are present in an iPhone 16?

A new battery.....for less than $100....will keep your phone running nicely for another 2-3 years.

But, it's your decision.

Mobile model selection is an individual choice based on many parameters. Such as Money, Feature, Status, Aesthetic, Utility, User-Friendliness, Availability, Warranty Coverage, Service Support, Resale value, Technology and many other factors. Choose yours judiciously!

You can choose yours, based on your need. Go through the tech specs of all the iPhone models in one place, here --> iPhone Tech Specs - Apple Support And iPhone - Shopping Help - Apple