An audit engagement is the most comprehensive report you can obtain. With an audit engagement, you have the highest level of assurance in your financial report. A review engagement is less intensive and provides limited assurance as compared to an audit.
Since the audit is more comprehensive, it involves more work and testing to gather the audit evidence as support for the final report. As a result, it is more expensive than a review engagement. The decision between whether to do an audit or a review will likely be determined based on the needs of the current and potential future users of the financial statements.