Currently the Gristmill and Dutch Windmill do have the same:

• Production speed (all recipes)
  
• Build area requirements

The only difference is that the Dutch Windmill is slightly more expensive to build.

I would recommend following adjustments to balance it:

1. Half the production speed
Due to no additional production requirements like mechanical energy, there is no need to build the Gristmill at all.
Make the Dutch Windmill production speed half the speed of the Gristmill, so its worth to consider spending more build space or just plug power.

2. Change build cost
Since its quite similar to the Windmill, i would recommend to add paper as build requirement.