After using the 14PM since launch and being very much in doubt whether I would get the 18PM, the Duo, or wait another year, I ended up deciding on the 18PM in Glacier.
My 14PM could certainly be used for at least another year, but its battery was unusable. the phone would heat up like crazy for no apparent reason and by the middle of the day I already had to charge it to last until the end of the day. This is without a doubt the highlight of this upgrade for me: the battery. I've never had a phone with a battery that lasts as long as the 18PM. With relatively heavy use I get about 10 hours of SOT. With more moderate/light use it easily surpasses that.
Besides the battery, having Apple Intelligence is also a bonus for me. I use the text/search tools quite a bit.
Other than that, there really isn't a huge difference in day-to-day use, in my perception. I hope to keep this phone for at least 4 years, as prices are already at absurd levels.