A very short comparison

Compared to m0n0wall, it (i.e. pfSense) has many many more features. That proves problematic for very basic systems, like the soekris net4501 which only has 64mb of ram. m0n0wall runs great on that platform, but pfsense requires 128MB of RAM so its a no go.

m0n0wall does have simplicity going for it, as well as security. Simplicity is nice in many ways – fewer things can go wrong, etc., and with no ssh or servers other than the webGUI which can use SSL, you’ve got a tight box – even the console is very limited.

There’s more detailed information on the differences and similarities between pfSense and M0n0wall on the FreeBSD systems page.