I don't really know to be honest.
I always try to stick with the apps/sites that pay me with actual cash, as its versatility is obviously much greater than other formats. I only use two that don't pay in straight cash and that is Purkle (which I use purely because it pays extremely well and has a fairly decent volume of surveys available) and Panel App (which I use because there is no real reason not to - it is effortless).
Having said all of that, if earning google play credit is your goal, I think Google Opinion Rewards is the app for you. As far as I know, it is the only app that pays in that format and even if there are others, I doubt they could compete with Google. Google can literally set any price it wants to beat the competition. I doubt anything could offer more Google Play Credit than it.