What is more important between Voltage and mAH consumed? Do I need both of them displayed on my OSD screen?

They are both useful indicators for OSD, and they are somewhat related too.

Voltage is like a percentage of capacity left (lipo total capacity - mAh consumed), but the problem is that voltage tend to fluctuate a lot depending on the load (how much current being drawn), the quality of the cells and the age of the battery pack (which affects cell internal resistance).

That's why it's sometimes more reliable to have "mAh consumed" displayed on the screen. When "mAh consumed" reached 80% to 90% of your LiPo capacity, your LiPo battery should be close to empty and the voltage should be at around 3.5V per cell already. But the downside of "mAh consumed" is, it won't work unless you use a fully charged pack. you have to know before taking off how much capacity is in the pack to know the capacity left in the pack during flight.

So you should really have both values displayed in your OSD to be safe...

I wrote a related blog post before:
https://oscarliang.com/monitor-measure-b...arm-drone/