What is the preferred impression for removable?

Prefer a Scan... we dupe the model regardless.

We dupe the original whether printed or impression. The dupe, being stone, is used for articulation, set up and process. The original is used for the final check.

Therefore when we have a scan we know the final print model will not break, or chip, as will stone. Nor does it have any pulls or bubbles to contend with as does traditional impression material. 

Scans can have voids and artifacts therefore we need a perfect scan otherwise they too will need those areas filled or removed increasing the inacuracy of the final fit for the patient.