Replaced my launch Product Red XR with a 15 over the weekend after the further price drop to £699. Needed to also switch out my Watch Gen 3 with a new SE but bit the bullet and did it all in one go (needing to also upgrade my watch was one of the things holding me back initially).
Went for the black colour in the end which looks basically identical to the Space Black Macbook Pro I also got (yes, my wallet hates me now..).
First impressions and I am loving the change. Initially I was hesitant after testing my wife's 15 against my existing XR in day to day usage as there wasn't that much of an obvious difference. However, having used it properly for a few days I can say its the little things that quickly add up into a noticeable change. Little things like the loading wheel when adding animated images into messages is no longer present (I just thought it was network related - it may still be to a degree but I haven't see it once yet with the 15), and app switching animations being that bit quicker. Again nothing earth shattering but it all adds up. At a guess I can only assume the 3GB of RAM on the XR is starting to become a bottleneck with the latest iOS.
Also, and this might just be a placebo, I am sure the XR's battery is taking a bit more of a hit during the day from the latest update, like 2-3% more every couple of hours. No change in battery status (still 87%) but just another little thing I noticed.
The Watch on the other hand is like a night and day difference in actual use. The slight change in what the buttons do is taking a bit of getting used to (maybe just settings I need to change), but the responsiveness and ability to track my sleep schedule are real game changers for me. I did find the receipt for the watch 3 from when I bought it (used without straps) - Aug 2018!
Overall very pleased and hoping this one lasts as long as my XR/Watch 3 combo.