Best formbuilder plugin ?

I’ve heard the Gravity Forms plugin mentioned
http://www.gravityforms.com/

But in the past I have used the free plugin
FormBuilder
http://wordpress.org/extend/plugins/formbuilder/stats/
It seems to work well enough and has a broad user base.

I wonder if anyone has used both of these plugins
and if there are any reasons why one should choose
the paid commercial option?