Pistols and Rifles in Dead Frontier are basically the same thing. They both cause the same amount of aggro when fired, but not much. Pistols uses handgun rounds which are cheaper. Rifles uses rifle rounds which are more expensive. Rifles have higher accuracy than pistols but some pistols especially "revolvers" have higher critical hit. Some rifles as you upgrade your rifle skills the rifles you by like 577 REX (which requires 110 rifle skills) does alot of damage on zombies. But it gets expensive.
So as I had said pistols and rifles are basically the same so DO NOT UPGRADE BOTH PISTOLS AND RIFLES SKILLS. Its best to choose wisely which one you are going to use in the start of the game.