Taken from the Player's Handbook - Fighter's Fighting Styles (p. 72)
Dueling When you are wielding a melee weapon in one hand and no other weapons, you gain a +2 bonus to damage rolls with that weapon.
Great Weapon Fighting When you roll a 1 or 2 on a damage die for an attack you make with a melee weapon that you are wielding with two hands, you can reroll the die and must use the new roll, even if the new roll is a 1 or a 2. The weapon must have the two-handed or versatile property for you to gain this benefit.
When wielding a versatile weapon, is there any benefit/downside to taking Duelling Over Great Weapon Fighting? Are you required to take Great Weapon Fighting when you are using a 2-handed weapon?