i like 9 x 18, i've been shootin my mak for 5 years and i'm not afraid of ammo scarcity. even if we quit getting the cheap stuff from china, russia, etc, and where S and B's stuff comes from, Fiocchi has some GREAT FMJHP's for the gun. ( i think it costs, get this now, 12 or 13 dollars for fifty rounds of it, lol, as opposed to the 5 or 6 bucks for fifty rounds of ball ammo). its true you could get a barrel and then have both, but...i like the 9 x 18.
with regards to the safety issue, i think slam fires and that drop fire stuff is kinda overblown, sure it may happen, but if you keep your gun clean, its kinda a remote possibility. I do recommend the Makarov Book, or some other manual.
have fun!
by the way, the makarov i have is a single stack, hmmm, its just preference, i like it better than the double stack kind.