I've been flying with an iFly 720 on the left side and an iPad mini on the right side for a couple of months now. While it's not an Android tablet, there are so many different Androids choices, that it's probably still a fair comparison.
The iFly is MUCH easier to read due to the brightness and contrast.
The iPad mini is larger, a plus for reading, a minus for fitting in the cockpit.
I addressed the power issue for the iFly with the external battery. The battery powers the iFly and has an on/off switch which I use to turn the iFly on and off. The battery in turn is charged by the aircraft power buss (12V in my case).
I consider the iFly 720 to be primary and the iPad a back-up. The iPad is easier to remove from it's mount (RAM) for my co-pilot to zoom around in, finding alternatives or other interesting things to see, etc., while the iFly stays "on task".
Hope this helps.