AR-15 has a two position safety. (safe and fire)
M-16 has a three or even four position safety. Safe, fire, Either full or 3 round burst, and some( very few have both burst and full like an H&K)
Other than that both are modular and can have stock, grip and upper swapped out to meet the shooters needs or wants. Like Leggo's for grown ups. Look it's a rifle, Click click click, it's a carbine, click, click its a varmint rig, click it's a squad auto matic belt fed. Click , it's a pistol caliber SMG, Click, wrench what have you and......It's really up to your imagination.