I've been an IT professional for about 25 years. All computers made in the last 10 years started out as pretty good performers. Today's ultra fast will be an also-ran in 5 years and not everyone can afford to upgrade constantly. For the vast majority that don't upgrade regularly Norton makes decent computers into pigs. Just my professional opinion. Avast! and Malwarebytes can run concurrently and both have free versions for home use. Their definition files are the same for free and paid programs so they will detect the same intrusions. The paid versions offer more ability to configure but don't always offer more ways to intercept intrusions so I've gone with the free versions on my home computers for years.
We use Symantec Endpoint Protection at work and it's pretty effective but I think Avast! and Malwarebytes are more agile in their response to new bugs and in their predictive programming that detects new undefined threats. We often have to wait 24-48 hours to get patches for newly discovered threats with SEP.
AVG seems to be about as effective as Avast! I am not a fan of Norton because it does slow down otherwise perfectly operating computers that may not be top of the line, latest and greatest with tons of RAM and SSD hard drives. I put Norton on my home desktop computer a few years ago and took it off within days because of the performance issues. Avast!, Malwarebytes and AVG don't consume a lot of resources and have had no perceptible impact on properly operating machines even if a few years old.
Running multiple antivirus programs is always a bad idea because each will likely detect the definition files of the other as viruses. One antivirus program and one anti-malware program can be run concurrently.
Apple products are not immune to bugs and as they become a larger portion of the market they are also a bigger target for virus and malware authors.
These are my favorites from the past few years:
Antivirus: www.avast.com
Anti-Spy/Ad-ware:
Super Antispyware: www.superantispyware.com
Malwarebytes: www.malwarebytes.org
Spybot Search and Destroy: www.security.kolla.de
Lavasoft Ad-Aware: www.lavasoft.com
David