I sold my S4 but have not noted anything appreciable re: brightness. As for battery, 6.1b1 is very satisfying and no discernible difference from my watchOS 5/S4 combo. I have AOD and the noise monitoring feature enabled as well as cellular. Yesterday was comprised of around 750 calories burned just wandering/walking/shopping and a quick 20minute indoor bike spin (phone not linked, transmitting audio over LTE and connected to my powerbeats pro). I also drove 3 hours with turn by turn with the directional taps going and then just standard notifications. Again, 47% when I placed it on the charger at midnight or so.
A few days earlier, I did a 7 mile outdoor run with GPS, music streaming over LTE to my PowerBeats, sunny bright day so when i checked my stats the screen was pumping brightness, and then went to work and did my usual and I still had some (limited) battery to spare after 7am to 1:30am or so.