You have 2 choices: Crosso and Click
The best choice is: Crosso, which is simple and has steel hooks that
are almost indestructible and yet cheaper to manufacture.
You may be interested in the Click system when:
1. The fixed hook track in the Crosso system does not fit to your rack. In the Click
system the hook track is adjustable.
2. Often, when traveling by train, plane or by other means, you may be carrying the panniers by 
hand. In that case the metal hooks may be inconvenient. In the Click system you can easily unscrew the hooks, which makes carrying the panniers safe and convenient.