Multisite Custom Forms: Gravity Forms Vs Formidable?

Hi All,

I have clients who are on Multisite asking about custom forms. The requirement is we must be able to include addons for mailchimp and other custom purposes. I've narrowed a long list of options down to 2.

From your experiences which one is better and why?

http://formidablepro.com/

or

http://www.gravityforms.com/

I've noticed Formidable is a once off fee where as gravity forms is yearly (I need developer licenses). Is there any reason why I shouldn't go with the more cost effective Formidable Pro over Gravity Forms?

Any recommendations based on personal experience would be appreciated.

Cheers
Billy