As above, newly setup watches tend to use more battery the first few days, and major x.0.0 releases seem to tend to have issues too (I like to wait for the x.0.1 release
)
One clarification - per the specs, the screen brightness is identical from S5 to S6 - both 1000nits.
The difference reportedly is that the AOD of the S6 has a higher brightness range up to 2.5 of the S5. My understanding is that this comes into play only when in a bright environment (outdoors/sunny) where the S5 AOD would max out yet still be difficult to read. I think (hope) the AOD remains at normal levels in darker environments.