Immunet provide protection using internet connection only (uses virus signatures stored in the cloud) , i.e., no offline protection and any type of behavioral detection. If you get onto their website, you will find that they advertise feature "BASIC DETECTION AND REMOVAL" thus it is recommended alongside of your main antivirus.
Bitdefender has one of the best signatures (offline signatures) and have good other features too like Active threat control, Ransomware protection, etc.
Thus, I will recommend to use Bitdefender to use. If you feel to try Immunet then you can do it alongside Bitdefender as Immunet is more secondary scanner option.