Preference depends on what you get accustomed to and want to do. Just be consistent. If shooting with a group or spotter be sure you are all accustomed to the same system (speak the same language). If you have no experience I suggest MIL over MOA as metric system is easier to calculate quickly in your head as long as you are familiar with the metric system. If you, like myself grew up with 1/4 inch clicks at 100 yards using the MOA system then I see no point in trying to change what you are accustomed to. Just don't take on trying to convert one to the other right away as that gets disastrous especially in a high pressure environment until one has some experience. For example if you are using MOA be sure you range with yards and if you are using MIL use meters exclusively. Eliminating conversation will help eliminate errors from conversation while getting accustomed to which ever system you choose. MOA is almost exclusively used by F Class and benchrest shooters because of of the finer adjustment available with most MOA optics but that is not necessarily desirable if quickly dialing dope on the run. In summary what your choice is depends on what you want to do what you and your constitutes are familiar with. Do your homework then decide what works for you.