should I update 14.8 or 15 iOS

Yes! for the first time ever you have a choice. 14.8 and 15 will both be updated with security updates for some time to come.

If you have iPhone 11, 12, 13 there are new features in 15 you might like.

Each iOS update will add security and bug fixes. It is recommended that you update the software.

Understood, but my phone says my system is “up to date”. So does that mean updating from 14.8 to 15 is optional at this time?