2 seconds will almost always be better. Ideally, you'd use a remote shutter release which allows you to vary the time depending on the number of fireworks going off. Two seconds of the single "when you wish upon a star" firework is a lot different than two seconds of the grand finale! The lighting of the castle changes during the fireworks, too, so you can't even depend on that.
Try these settings as a starter:
Manual mode
ISO 200
Aperture: F8
Shutter speed: 2 seconds
Use the shutter speed to adjust how long the trails are.
If the firework trails seem too bright, try setting the aperture to F11.
If the firework trails seem too dark, try setting the ISO to 100.
You will also want to set the camera to manual focus. Try focusing on the castle then go just slightly past it towards infinity. Take one or two test shots to make sure that the castle is still sharp. You are focusing a little bit past it because most of the fireworks are far beyond the castle.
Good luck!