Both dramas feature time travel to a different era.
In Tunnel, granted it is just from 1986 to present.(still a big culture shock)
In Faith it is to the Goryeo era from the present.
Both bring a little star power with their Brave and Manly leading men.
Faith's cast is lead by Lee Min Ho playing a General of the army and bodyguard to the King.
Tunnel's cast is lead by Choi Jin Hyuk playing a tough Detective who is not afraid of tackling a dangerous criminal. He is also a respected sunbae for his less experienced partners.
Both have a lead who is faithful while longing for their loved one after being separated by time.
Both are directed by Sin Yong-hwi
Both are really good dramas worth watching.
Where they are different:
Tunnel has a serial killer crime story and slightly dated outfits.
Faith has political intrigue, medicine, and of course the long hair and historical dress.