Leopard Server Firewall vs Hardware Firewall

We currently have a network consisting of a couple of Windows 2000 Active Directory servers and a Mac OS X Leopard Server running Open Directory, iChat, and Web. Both platforms are used as file servers, too.

At the moment, we have a NAT device and hardware firewall separate from the servers, however, I am looking into setting up a Leopard server for VPN, and was thinking about running NAT and Firewall on that server, as well.

So, basically what I'm curious about, are the pros and cons are of using a separate hardware NAT/Firewall device versus using the tools integrated into Leopard.

I have not been able to find a definitive answer, and am curious about what people think.