A notarized will is a will that is notarized by a notary officer.
Typically, to make this type of a will, an attorney collects necessary documents depending on the contents of the will, prepares a draft, arranges the wording with a notary, and accompanies the client to the notary office to finalize the will.
A notarial will is regarded to have following advantages:

• Reliability: Because a notary and an attorney are engaged in the creation, the accuracy of the will’s contents is guaranteed.
• Security of Storage: Because an original copy of the notarial will is kept in a long-term storage by the notary office, risks of loss or alteration are significantly reduced.
• Ease of Execution: Because the will's contents are clear, inheritance distribution and other inheritance processes go smoothly. Especially, in Japan, a notarized will can be executed without any court proceedings.

A holographic will is a will that is handwritten by a testator. It is also called a handwritten will.
A handwritten will can be created by oneself without a visit to a notary office, however, it is considered to have some risks.

• Legal Requirement: A handwritten will must meet certain legal requirements, and failing to do so may nullify the will.
• Loss or Alternation: If the handwritten will is not stored properly, there are concerns about loss or alteration by others.
• Complicated Procedure: To execute the handwritten will, a procedure called 'probate/ Kennin (検認)' must be conducted in a family court. A probation requires identification of whereabouts of all legal heirs, which would be cumbersome especially in foreigners’ cases.

Considering its reliability, security of storage, and ease of execution, I advise that the client make a notarized will.