Both dramas are about two people being in fake relationship (or contract relationship) to achieve their goal. In both of these dramas, male leads are rich men from higher society (both are actually hotel owners/managers) and female leads are average women (a teacher and an office worker).

Both are war movies set in 1950 during korean war and both are stories about bravery and sacrifice.

71: Into the Fire is based on real events while Tae Guk Gi is a fictional story

both are set in past and are about past and current friendships.

Sunny is about group of friends that decide to meet after 25 years of not being in touch and are reminiscing about their high school years while Reply 88 is about life of group of friends living in year 1988