Both are very intense historical dramas that will keep you hooked.
Also, Jang Hyuk plays a main role in both

in both, a character gets pregnant after a one-night-stand

both also feature a contract marriage where the couple falls in love later

both take place at a hospice
both have an overall sad vibe
similar cinematography
in Chocolate the leads work at a hospice while the FL in The One and Only is a patient