Use the SSN. It will help you if you ever get an attachment or levy or other legal service on John. All of his accounts, including the business accounts, will be reached by such documents and actions.
If you pay interest on John's business NOW account, it will correctly be reported under his SSN (John has to include his business income on his personal 1040).
If you use TINs for aggregating cash transactions for CTRs, you should be aggregating John's personal and business account activity (since they are all owned by one person), so use of the SSN is your better choice.
It's not wrong to use the EIN for the SP account, but it's just more correct to use the SSN.
John's SSN is also the ONLY TIN you can use to correctly complete your CIP requirements, because John is the account owner (not the business).
John S. Burnett
Fighting for Compliance since 1976
Bankers' Threads User #8