Declarative Processing - In practice, declarative rules are evaluated automatically when needed to satisfy certain conditions. These rule types differ in processing to offer you specific meanings of "often" or "as needed." The primary benefit of declarative processing is that the system, rather than the developer, controls when computations are processed. Relationships between property values can be expressed directly, without the need to create activities and design the application to run the activities often.
Data Transform - Use a data transform to define how to take source data values — data that is in one format and class — and turn them into data of another format and class (the "destination" or "target").
In general, data transformation involves both mapping data from a source to a target, and performing any conversions or other transformations on that data that might be required to achieve the intended mapped results.
When Rule - When rule evaluates the expressions or situations configured and then returns boolean result (true/false) accordingly.
For your use case:
1- If you are using different properties to calculate the value of property, then you should use declarative rules.
2- If there is a use case where you are copying the data from some other property to the property mapped to the UI field and displaying that data, you should use Data Transform.
NOTE: Use of these rules depends completely on the requirement and the use case scenario. There might be possibility that you will be able to use any of the above 2 rules to populate the value of a property but which one will be the best approach depends on the use case and configurations done.