So you’ve got two good options … Smoothwall and PFSense. Both are free, both come with pretty web based user interfaces, both are firewalls … It’s really just up to you. Make a choice! Really so long as you configure either properly, you should be fine.
Smoothwall is an open-source community effort firewall. It’s a free firewall that includes its own security-hardened GNU/Linux operating system and an easy-to-use web interface. It appears to be consistently updated, and what I like about it is that it uses Snort to keep it’s rules automatically up-to-date and deny known attacks.
The pfSense project is a free network firewall distribution, based on the FreeBSD operating system with a custom kernel. pfSense software includes a web interface for the configuration of all included components. It has the ability to generate pretty graphs (good for reporting!) and there is a good chunk of plugins to extend the functionality of pfSense directly listed/added from the web interface. pfSense is also consistently updated, and has a large community behind it.