The defacto standard for comparing one database solution to another is the TPC-C benchmark. The latest definition of this benchmark can be downloaded from http://www.tpc.org/tpcc/default.asp along with existing benchmark result sets.
While it is unlikely you will run the full TPC-C at your location, the test definition should provide the foundation for developing a set of internal benchmarks designed to identify performance differences on a reference set of hardware with respect to your organization's specific requirements for transactional throughput.
Make sure you download the cost of ownership documentation as well, for this can provide a standardized method for evaluating the two solutions.