Here’s my thought process. Idk if I can actually answer your question for you.

It would cost you 80 USD to have the battery swapped out in the 13. If the 13 is still good otherwise, you could keep using it another year or two.

That being said, look around eBay or Facebook marketplace etc and see how much your phone would fetch if you sold it. Apple trade in won’t give you as much as you would get otherwise. But being that it’s not a 13 pro, it may be worth looking into selling, and if you get a good price then why not upgrade. Providing you can afford the upgrade and don’t finance it. My rule is that unless it’s something like a car or a house and you can’t afford buying it without financing, then you don’t NEED it.

I gave my wife my 13PM (still has 95% battery health) and I picked up a 15PM for myself. It’s nice, but the only things I like more are the camera, USB-C and the weight of the phone. If you are coming from a regular 13, then the screen will be a nice upgrade, too. Oh, and 256GB minimum size for storage would be another reason to upgrade (plus, you can always easily plug in more storage via the USB port without needing a dongle).

If you have a regular 6.1” 13, then the 15PM is a worthy upgrade, especially if you like to take video and pictures.