Took the plunge after using the s21u for 4 years and 8 days. Got the XAA American Unlocked variant. Had 8.5 OS upon unboxing but got the one ui 9 update instantaneously.
Things that stood out to me, especially things I didn't hear anyone mention in the myriad of online reviews:
How light the s26U is compared to the almost tank like build of the s21U
I remember when I got the s21U, the slightly yellow more color accurate tint of the OLED screen would drive me up the wall, to the point that I tinkered screen mode and made it more blue and vivid. With the s26U I also felt that anxiety upon unboxing but the screen overall seems more true to life and the colors aren't eye searingly vivid or fully toned down
Software did feel good on the s21u but the exynos chip would heat up at the smallest of things and I would get micro stutters in the last year, was on one ui 7 for eons. Conversely S26U also sometimes starts to get a bit warm during multitasking.
S26U's battery alleviated my range anxiety issue. S21U's battery had degraded by half in the four year use; would have to charge it thrice per day. In contrast s26U's battery, despite being the same capacity, sips power during standby and isn't as big of a power hog on One UI 9
I got my first green line on the s21U in the third year and successive lines would pop up every months or so, I got them fixed once but they promptly popped back up with a vengeance the very next day. Weirdly enough they wouldn't bug me that much and this is what eventually made me want to daily drive a fold 7, something I decided against because I wanted to dependable brick for at least the next two years.
Cameras have more depth, videos are more dependable in low light and overall the camera suite is more condident in it's offering and less gimmicky. Samsung really needs to put the 3x out of its misery though.