What is the best state to leave an Android device in? I have a Moto E phone. I use it as a tablet to run apps, not as a phone, so I do not always need to leave it on.

Is it more likely that the device will last longer if I shut it off whenever I am not using it? Or is it better for the hardware of a phone to leave it constantly on, in sleep mode?

And is it better to leave it constantly charging, or to only charge it whenever the battery is below 100%?
Looking at it from a battery point of view


Leaving the device in sleep mode is preferred , unless your usage is very infrequent
In addition, you can use Greenify (see greenify wiki) and Wake Lock Detector as explained here for unrooted devices
As far as ideal charging limits are concerned, you can see this and decide what suits you best Ideal charging / discharging percentage for maximum battery life?
Also see Does constant charging harm my Android cellphone?
Switcing off Wi-Fi and Bluetooth 


Once your battery consumption is taken care off,  there isn't any concern in leaving it switched on in sleep mode