Are the two SATA bus interfaces in the 2010 Mac mini equal in speed? I want to replace the DVD in my old 2010 mac mini with a SSD but first I would like to know if the two SATA interfaces are equal when it comes to speed.

I would prefer to keep the harddrive on the first interface as it is much easier to change it than is to change the one in the DVD location.
You can answer this question yourself by running Apple System Profiler/System Information and noting what it reports under the "Serial-ATA" column.

Here is what the report looks like on my MacBook Pro.