Ive heard some say that the only diferance was that they are visually inspected befor packaging... but I don't buy it. Ive burned more than a case of them both under powder, and in a test fixture, and in every case the Ms have had lower ES and SD. With the price differance between them so low... I wouldn't care what I was assembling the ammo for weather its pdogs at 1500, or turtles at 30... its gonna be match. If youre buying them locally, its worthwile to buy a 100count box of 2 or 3 lot#s to see which is better. Usually it won't make any differance, but ive seen them range from good to great. Ill spend $6 more testing for great primers anytime. Additionally... again if youre buying locally, make sure its packeaged like the one on the right.
The left is the older packaging... and consequently has been setting on the shelf for a while.