I prefer the 30's, mainly because they are a bit handier to carry around. Both magazines are extremely reliable. The 42's are a bit longer, and make going prone slightly less comfortable. Everyone should have a few of each though. For when I am shooting squirrels while walking the field, I like the 42 for the amount of ammunition I can carry on the gun. Also with 3 spare mags you have another 126 rounds on tap.