Derby or MySQL or…?

For what type of requirements would you choose Apache Derby (or Java DB) over MySQL (or vice versa)? I looked around and people just compare the two but no one talks about when to consider each one. I am developing a web-based application using Glassfish + Java/Restlet + MySQL.

I expect around 100-200 users for this system with a load of about 30-50 concurrent users at a given time - mostly.

I was told to look at Derby if I wish to make the web-app downloadable/distributable. But is that the only reason why I would use it? Is it suitable for web-apps? Has anyone used it? What has been your experience and when do you choose one over the other? (Most discussions of the comparison predate MySQL v5 when it didn't have support for stored procedures, triggers etc., But that is no longer the case).

I can understand a standalone DB server model with a web-server sending the requests, but how does this model change with an embedded db? Or does one default to using Derby in the network configuration?