This is a good question. I have beaten both in the Arcade. Mortal Kombat 2 took me about and hour and a hafe. But mortal kombat 3 only took me about 30 minutes. Mortal Kombat 2 is hard from the first guy you face wile in MK 3 its always around the 4th guy that it starts getting hard.
So my rulling is. MK 2 is longer.