Mils are just an angular measurement there are 6280 mils in a circle. There are 21,600 minutes in a circle. The mil dot scope has dots for every 1 mil that can be used for ranging. Newer reticles are usually hash marks at 1/2 mil. There are also MOA hash that are based on minute of angle. All can be used for holdover without dialing drop.
At 100yds a mil subtends 3.6" while a MOA subtends 1.047".
As for use its really personal preference. But beware of scopes that have mil reticles and MOA turrets. That is nothing but confusion.