Welcome to Nothing!
Nothing OS is exciting, fresh and unique. Samsung’s OneUI is extremely customisable nowdays though goodlock so you will lose some degree of customisation but you will experience new ways to redefine your homescreen and lockscreen with Nothing OS.
If you play games then, unfortunately, you will realise that there are games running less smoothly on Nothing Phone 3 (60Hz) than in S25U (120Hz). The rest of the applications and OS are smooth and everything feels lightning fast.
I believe that overall camera experience is better with Nothing Phone 3. I am proud of most of the photos I captured with Nothing Phone 3 in Venice than the ones I captured with S25U in Barcelona, the ones in Barcelona have much more noise in every single scenario.
Nothing Phone 3 does well with both battery and brightness outdoors so no worries there.
The one thing you have to forget is monthly updates. Patience is a virtue with Nothing OS in this matter.
Enjoy!