the first one I will buy you drinks ( in general any drink juice, coffe , tea etc ) 음료수 here means beverage the second one tends to be a drink alcohol ( suju / beer any drink contains alcohol if you let it plain like this without mentioning any specific drink name before 한 잔 ) but you can say it with any drink too you just put the name of the drink before 한 ( or the number of the cups ) and 잔 .. this one contains of 한 means one and 잔 a counting item means cup