Webflow is an web-specific IDE.
It's great for production work where you are explicitly targeting the web --- we've actually pointed some of our Kickstarter backers to Webflow after they explained their needs and we realized it'd be a better fit for them.
Subform doesn't target a specific platform. Its focus is to help folks explore layouts and design ideas easily. If you are familiar with industrial design, an analogy would be that Subform is somewhere between SketchUp or Inventor/SolidWorks, and Webflow is closer to a CAM tool.