m16 ar15 m16a1

Hi, I'm thinking of getting an AR15 (finally) and I saw an add for brand new, set back to semi auto (due to the law in my country. yup they suck...) M16A1 20'' barrel for 1600$. Good idea or should I go for the more commen M4 versions? I'm a complete newby on the AR15/M16 front. Brgds, Robin