Sorry no I mean is the AOD mode specifically brighter?
Don't buy the S6 for the "brighter" AOD mode. It isn't noticeably brighter at all. I've tested against my S5 and there is literally zero difference. If there is a difference, it must have to be specific watch face as the ones I tested were exactly the same. A change here may have occurred on paper, but not one that is noticeable to the human eye.
With that being said I'm not saying don't update to the S6, I'm just saying don't use that as a reason because you won't see a difference.