The choice between the editors depends on the user’s specific needs and requirements.
You should consider continuing using CKEditor 4 if the compatibility with old browsers is a must for you or if features that are essential for you are not yet available in CKEditor 5. However, being a totally new editor, with time CKEditor 5 will have more and more features developed and available for the end users to benefit from. At the same time, we are determined to continue the CKEditor 4 development and maintenance for some good time still. Bug fixes and browser support will be provided until January 2020 and security bug fixes and an extended SLA support until January 2023. Giving CKEditor 4 users enough time to make a move towards CKEditor 5.
If great user experience and clean UI are your priority and the features currently available with CKEditor 5 Builds (Classic editor, Inline editor, Balloon editor) are sufficient for your use case then you should consider using CKEditor 5 Builds.
To find out more about CKEditor 5 Builds refer to the CKEditor 5 Builds documentation.
If you wish to create your own text editing solution and have full control over every aspect of the editor, from UI to features, and the possibility to enable real-time collaborative editing inside the editor you should consider CKEditor 5 Framework.
To find out more about CKEditor 5 Framework refer to the CKEditor 5 Framework documentation.