Antivirus vs. Firewall Protection

A Firewall monitors open connections and like an antivirus can monitor email retrieved with mail clients like outlook, for worms and such, but it does NOT act in place of a virus scanner. It just looks for unusual file attachements and the like.
Firewalls also block unwanted inbound and outbound traffic or connections via IE or any other program that connects to the internet for whatever reason, and you can allow or deny any program access to the internet.
You can also configure a firewall to block cookies, ads, popups and websites that you specify.

An antivirus analyzes any file that comes through your system for all known viruses in its virus definition database, and cleans, quarantines, or deletes them per your choices of settings.

Antivirus would have no compatibility issues with setting up internet access via a home LAN, but a firewall would have to be configured some, to allow the computers to access each other and the internet.

If there are any performance issues, they would be negligable, but a tiny loss in performance is a small sacrifice to make, as opposed to the alternative.

I myself, use both, and would recommend the same to anyone.