Best units of measurement in Unity

Quick simple question, I am about to try and work out calculations and stuff for my game (first person shooter) was was curious to know what units I should use in unity to make things easier, or if it makes no difference, for the following:

Bullet Speed:
Feet per second vs meters per second

Bullet Energy
Foot Pounds vs Joules

Bullet Mass
Grains vs grams vs kilograms?

I know computers being computers means that I can put it in however I like then when I create the script or whatever I can...

Best units of measurement in Unity