Basically, these are two different options to customize SharePoint forms, change the default layout, but also configure business logic in the form. Both ways each form (Display, New & Edit) separate.
- Rich Forms:skybow rich forms has been around for several years already and offers several functions to customize the so-called Classic Lists/Libraries and their forms. Classic Forms are still available on all platforms (Server 2013/2016/2019 and SharePoint Online).
A detailed documentation of all functions can be found here: Rich Forms Documentation
- With Modern Forms skybow offers a possibility to modify SharePoint forms of the newest generation (Server 2019 as well as SharePoint Online only). Modern Forms are fully responsive and can perfectly be used on mobile devices.
The functionality is still limited compared to Rich Forms.
Please find a feature comparison sheet here: Download Feature Comparison Sheet.
When should I choose which option?
If you want to deploy the solution to a SharePoint Server 2013 or 2016 environment, you have to choose the skybow rich forms option.
In any other case, it is primarily related to whether your site is already running using the new experience or still classic.
If one of the main criteria is to use it on mobile devices, then the Modern Forms option should be chosen, because this solves this requirement with no effort. As well using the Classic Option there is a solution for mobile devices, which can be read here: SharePoint-based business apps: high-performance, good looking and responsive? Sure!
It is also possible to use the forms in Classic mode within a Modern List, but leave the list views in Modern mode. The other way around is not possible.