FreePBX is more feature-rich than AsteriskNow. However, AsteriskNow is the closest thing to an “official” GUI for Asterisk, as it’s a Digium product.
Other than that, there is also some basic difference in how both packages actually work.
FreePBX saves changes to a database and then rewrites Asterisk config files when you “apply changes”. AsteriskNow is actually writing changes to the config files.
My personal preference would be for an AsteriskNow that had the functionality of FreePBX…but we’re not quite there yet.
Perhaps you should start with one of the pre-built packages and try for yourself…pick any one you’d like. CentPBX, Trixbox, Druid, Elastic, PBX-in-a-Flash.
While FreePBX has a “better” interface I think Asterisk now is better because it works directly with the configuration files. With FreePBX there are loads of lines of code that are generated. Also it helps people that are new to Asterisk learn the system because with every addition and change to Asterisk they can see the corresponding code vs. FreePBX you will have a hard time figuring it out.