One thing for sure, FVS338 can provide fail over options: for example if you have two internet connections and one of them fails the other one will take over. Where on the other hand, I don't think Smoothwall offers this capability.
FVS338 offers VPN capabilities and Smoothwall doesn't. The FVS338 has a lot more advanced features that, Smoothwall doesn't have.
FVS338 has routing capabilities, that exchanges information between two different network segments. Smoothwall doesn't
I think FVS338 will be better suited for a business environment rather than a home environment. For a small home network set up, smoothwall will be more than enough.
For comperison purposes I think you should read the following links below.
http://www.smoothwall.org/about/express-feature-list/
http://www.netgear.com/Products/VPNandSSL/...=Specifications
Regards,
Infiltrator