L2OFF it is really nice when you do not modify nothing. The problem with L2OFF come when you touch some file and you provoke errors and bugs in your own server. You usally do not have the source code.
L2Java is not close to L2OFF speaking about the default systems such quest, damages, skills, classes, etc.... that L2OFF have, but you can customize many stuffs easier cause you have the source code. Having the source code depends on your datapack provider.