Dont get so hanged up in the name these programs have as they are years old and things have changed a lot
All virus are malware, but all malware are not virus
Malware = Malicious program (virus / trojan / adware / spyware / worms / ransomware /..... )
Avast detect all types of malicious programs
Malwarebytes does not target real virus, it also does not target script / doc / media files
Malwarebytes is certainly not light on system resources.
It is if you use the free version (no realtime protection)