Midas is Lapua's finest match ammo.
Midas L is slightly larger than Midas M and works better in North American and British match rifles. M seems to work better in Annies and the like. This is a generalization. Every lot of ammo shoots differently in every gun. Some rifles will print better with some lots the cheaper Master than with some lots of Midas.
If you are going to pay this much for ammo, buy one box each from as many different lots as possible, test them indoors or under dead calm conditions, and then buy as much of you can of the lot that works.