Windows Version Of hTC One (M8) is more efficient than the Android one.

Whenever we see spec wars between smartphones, a lot of people
forget to note that each operating system handles its respective
hardware differently. It would be unfair to compare a Windows
Phone to an Android phone on a spec sheet, as we do know that
Windows Phone requires much less power to perform just as well as
Android, and which is something that doesn’t speak highly of
Android. If you’re wondering just how power efficient a Windows
Phone can be when running on the same specs as Android, just
have a look at the HTC One M8 for Windows , and see for yourself.
The spec sheet of both devices currently claims that the HTC One
M8 for Windows can last almost twice as much as its Android
sibling. Official HTC listings claim that the HTC One M8 for Windows
can last up to 21 hours of active use, while the Android variant can
barely go for more than 12 hours on the same 2600mAh battery.
When it comes to standby time, there is less of a difference, where
the Windows Phone variant will last 15.5 days compared to 12.2
days on the Android variant.
We still need to run our own real-world tests in order to confirm if
these differences are that staggering, but yes, if battery performance
is your thing, it’s hard to not be inclined for Windows Phone.

Source: WMPowerUser