Difference Between NIS and Antivirus

My NIS renewal is coming up and I was looking to save a little money.  I have a Windows 8.1 PC with a late-model Cisco router.

 

The only differences between Norton Internet Security and Antivirus I could find were the following; did I leave anything out?

 

Parental Controls

Firewall

Spam Blocking

Defrag (Optimize) Scheduling

Startup Manager

 

The only feature of real benefit to me is the firewall, so basically I'm paying $30 a year for the firewall.  I'm a very cautious internet user, so If I were to downgrade to Norton AV isn't the combination of the Windows and router firewalls a sufficiently strong defense? 

 

I haven't made a decision and will stick with NIS if there is a compelling reason, so I appreciate any feedback.