I've had both of these but Ultra versions coming from an S10. Both seem massive compared to the S10. In fact, I almost sent my Ultra back because of this. The Ultras are also heavier but I kept the S21U because the battery life was way better. Am not a heavy user and would get around 2 days use although that did decrease over time.
I got the S23U a few months back and the battery life is amazing. I charge it every 3 days.
I would highly recommend the S23U but I saw that was out of budget. Perhaps interest free over a few years rather than 2 would help? After this model Samsung go back to their own chip so this should be a keeper for a good few years, 3 major updates and it flies so a great device.
The S21Ultra would be way cheaper but still have a bigger battery.
The screens on these are massive. I will probably always have a larger phone, now so an Ultra of some description is likely to suit you better IMHO.