mysql is a popular database system. I prefer derby technically because it offers no external dependencies. Because derby is a Java application I can pick up the pieces and move them to a different directory or a dedicated server. 100% pure is a big perk for me. Also as of right now Derby is the target platform.
I have not looked for a benchmark Derby against MySQL. However remember that a hive query might run for hours, a few ms latency in fetching Derby meta data is a much smaller order of magnitude. The above is opinion based on my deployment.
