Feat Second Chance (XGTE p. 75): The feat does not specify what happens after the attacker is forced to reroll. Does the halfling’s get to choose which roll is used? Does the attacker have to use the new roll (5% chance reroll is a crit)? Is the lower roll used? The attack is rerolled, thereby negating the previous roll, and using the new roll. If there was a choice between rolls, the text would say so.

— Eric Green (@quadhund) January 18, 2018

You're spot on, @quadhund.

— Jeremy Crawford (@JeremyECrawford) January 18, 2018