I don't put it on all day, but usually at around 10-20% (when those notifications pop up) an that adds an extra hour or so battery life. I usually get home with about 30% battery (same with the 6 when it was my daily driver) so I never need to use it most days unless I am testing the battery out for fun. I would imagine for a 6 user on iOS 9, it would be similar. The thing is, the 6 only has a ~5% larger battery. In real work use that is negligible. Your initial post would have someone thinking the 6 has 2x the battery capacity or something significant.
Both phones give the user "all day" battery with general use, at least for myself and others. I cannot comment as to what your uses are (for example, perhaps you don't use LTE or you do use bluetooth, or if you have an Apple watch those seem to suck battery life away - when I owned one, in my experience it did)
But again, you're not appreciating how small a difference ~5% is. They both offer similar battery life, is my point. The Phone 6 is not in some other tier of battery performance because it has a 5% larger capacity battery and more power hungry components.
Obviously your experience with the 6s has been different, so I can't say your experiences are wrong - there is a chance you have a faulty battery or there is a bug you are experiencing leading to battery to drain,
but what I am trying to get at is the differences are so marginal between the 6 and 6s between battery life (and those differences are offset by the more power efficient chips in the 6s) that you should not be getting wildly different battery life in real time use. They should be the same, or similar.
If you are getting differences, then its not because of the 5% smaller capacity, but something wrong with either your software or hardware.