Spam filter options for unregistered user to registered user email service
As an alternative for Mollom, perhaps Akismet could work? The service runs analysis on the comments/posts and determines whether they are spam from their content.

As an alternative for Captcha, perhaps Are You A Human could work? Rather than using wobbly text it asks you to complete a very simple game - causing less frustration to your users.

To properly answer your question: Given the two alternatives, I think the most efficient way to filter spam would be through Gmail, although it might violate their EULA.