The brighter display is more of an efficiency/battery benefit since a newer brighter display usually means it will use less power at the same brightness levels.
Officially Apple has never changed the battery life claims for the Apple Watch and under direct sunlight with the AOD all models even the latest Series 10 ones die rather quickly. My record was 23 hours from 100% to 1%/empty on my Series 10 with a particularly bright and hot summer day this year because of AOD despite using Airplane mode and never turning on cellular. Health is at 98%. I was shocked too and it was rather inconvenient since I was travelling and couldn't charge it for a while.
Turned the AOD off in the last few hours but it went from 10% to dead real quick regardless. Didn't even do any sleep tracking in those 24 hours either (I was awake). Normally I go from 100% down to 50% in 24 hours. Just goes to show how much of the power budget the display can use up.
Still, in terms of average battery life the brighter display should reduce power draw a bit for AOD users.