So for anybody still interested, I was able to test a 6.3 Pro this weekend and compared it in detail to my 12. There was no tangible difference. Yes, in certain apps (eg Maps) you see a few mm of extra detail in each direction but in other apps (eg Safari) I found virtually no difference at all. I think I realised why - its the Dynamic Island which is thick and low. When combined with the fact that most apps have a ’header’ section which has to accommodate the DI, this meant that the header was a decent way down from the top of the screen thus negating much of the benefit of the increased height. The remaining space for the body of the app has only an immaterial amount of extra space.
If the rumours are true about a smaller DI next year, maybe this will better liberate those extra 0.2 inches. Until then, I plan to stay with my 12 which is holding up great.
Edit: the difference between 6.1 v 6.3 may be more noticeable when comparing phones that both have DI but I did not compare.