Inferno ST-RR Gearing Question

Hello everyone.
As I am to finally get this done (after some stuff happening along the way),
I have some questions still on my mind.
Looking at the speed calculator and I noticed the following;
a 52/12 Spur to Pinion ration yields a theoritical Vmax of 37.69mph with a motor speed of 40700 RPM on a 5S battery.
A 52/13 spur to pinion on the other hand increases the Vmax a bit to 40mph and lowers the Rollout to 1.06:1 which is closer to the 1:1 ratio.
Which is the "best" one to use theoretically speaking?
Is there a way to calculate the running time on these settings?
I will try to avoid using the 50T spur gear as this is a metal one and I read that is better to opt away from metal spur gears in case the mesh fails.