The only similarity between these two is the body swap
The stories are totally different and have different genres

Both dramas have Body Swap
The stories somewhat turn out to be funny for both leads
Big depicts the teenage side of being a different person, But the day of becoming you both are adults

The stories both revolve around fighting for justice
There is Amnesia in Both
similarly, the background stories are really elaborate and keep you entertained

Both are romantic fantasy dramas.
There are 3 stages: 1- first encounter in heavens 2- mortal realm and hardships 3-a massive war
The leads are really different. One is reserved, yet the FL is so bubbly and happy.
There is death and amnesia in both.
The leads fall in love deeply, but are separated due to misunderstandings and wars.
There is magic, devils, hatred, love, anything you wish for!
Both are the best in the era

Both are romantic fantasy dramas.
There are body swaps in each of them( Albeit a bit different)
One of the leads is Quirky and hates the other one at first.
There is Amnesia in both.
Lots of battles and gods and demons in both of them.
Love between fairy and devil is much more elaborate and has different stories included, yet The eternal love is quite simple.
Romance is so good in both

The leads are really similar. They go through so many hardships to find their genuine love
There are gods and battles, reincarnations, evils, fairies, magic, everything you desire for!
Each clan has their stories depicted quite interestingly.
The romance is just marvelous in both( You will love them from the beginning :)
Both dramas show Amnesia as a major substory
There are different stages included: From immortals and gods, to demons, and mortal world. Even the mortal hardships and punishments give off the same vibes.

The stories are the same in so many ways.
Both are about CEOs and their dirty plots.
Leads suffer from Amnesia.

Both dramas show childhood friends turned into lovers with so many misunderstandings
Both are set in school days and show the students' family backgrounds and past.
Both have cute soundtrack selections
There is a love triangle in each of them
One has a strong unrequited love

Both dramas are set in College days.
Both dramas revolve around friends and classmates hanging out together ( They eat out together, go on trips, fall in love, breakup, etc)
Both have love triangles and unrequited love
Even the soundtracks are somewhat similar and give off the same vibe
The leads have been through hardships and have been raised in unfunctional families

Both are supernatural dramas
MLs are fantasy creatures with a dooming past and an impending future
FLs are simple humans
Magic is involved in both
A doomed romance between the leads

Both are supernatural and fantasy dramas
One of the leads is a supernatural creature doomed to live a long life
There is someone behind the magical lives that guides them through
Lots of magic in both :)

Both are fantasy dramas with the ML being a supernatural creature
The FLs are simple girls
Both have magic involved