They are eactly the same thing.
Certificate = Letter
From the IRS:
Complete and sign a statement on the Certification by U.S. Person Residing Outside of the U.S.certifying (1) that you are eligible for the Streamlined Foreign Offshore Procedures; (2) that all required FBARs have now been filed (see instruction 8 below); and (3) that the failure to file tax returns, report all income, pay all tax, and submit all required information returns, including FBARs, resulted from non-willful conduct.  You must submit the original signed statement and you must attach copies of the statement to each tax return and information return being submitted through these procedures.  You should not attach copies of the statement to FBARs.  Failure to submit this statement, or submission of an incomplete or otherwise deficient statement, will result in returns being processed in the normal course without the benefit of the favorable terms of these procedures.