Both types are designed and tested for secure cryogenic storage, so it’s largely a matter of personal preference. In an externally threaded vial (in which the cap covers the entire top portion of the vial itself), the underside of the cap is protected when unscrewed), which can minimize the potential for cross-contamination. In an internally threaded vial (in which the vial threads are inside the neck area and the cap inserts like a plug), the cap is the same diameter as the vial itself which can be helpful in certain types of dense storage applications.