It depends. Auto brightness can actually go brighter than what manual can. It aims to keep your phone at the same viewability. So if you're outside in the bright sun, your brightness will be super high. So if you're ok with a dimmer screen, manual could save power. Though if you then go inside, you'd need to lower the brightness else auto would save power. I'd say just keep it on auto and adjust to how you like it if it seems off. Samsung auto brightness learns how you like things set so eventually will always be where you want it to be
