You want to use pre-existing data in your forms, and aren't sure whether to use a cascading select in order to upload a CSV, or whether to build a hierarchy drill-down. The most important factor to consider when making this decision is maintenance.
If your data is static, or relatively static, you can use a cascading select. Remember that if you want to make any changes to the data in the CSV, this means you'd have to upload the new CSV, clone your form and publish it with the new file, and then publish a new Job with that form. You want to minimize having to roll out new Jobs often, so that field officers can work offline with the latest data available.
Example: Country > Region > District
If your data is dynamic, you should use a hierarchy drill-down. If you're adding new records to Salesforce that you want to be automatically sync'd to the mobile device without having to make any changes your Jobs or Forms, this data should be exposed to the user with a hierarchy drill-down.
Example: Program > Farmer Group > Farmer
If you have any questions, please reach out to firstname.lastname@example.org