What criteria will you use to evaluate these ? If you don't know yet, then you don't need to decide right now. Try to make your application as database-implementation-agnostic as you can - providing the appropriate wrappers, data access objects etc., and make this decision when you have all the facts to hand and you have to decide.
If you're using relational databases and SQL then the above shouldn't be too hard (using JDBC etc). Make sure you have plenty of surrounding tests so that when you want to switch between databases, you can determine that your application's functionality remains the same.
I ran into the same issue some time ago. I didn't know which database to go for, so my first solution used Derby (or HSQLDB?), and I was later able to switch to HSQLDB (or Derby ? Can't remember which solution worked) once I'd determined where I had issues (relating to performance) and which solution would really work for me.