I have tried both and I only wanted to know if one of them is to prefer?
Especially in preparation for OH3 it should be very easy to switch.
Does anyone know, which one uses less system resources?
The backup script won´t save the flows, am I right? I have to export them before doing changes like upgrade.
imo the build in rules should use less system resources. Did not test or monitor this, so take my statement with a grain of salt.
Not sure about that one, but I think so, since Node-RED is not a component of openHAB default why should the backup script work this way. I also did a manual backup of the flows.
Each one has their pros and cons for different use-cases.
I read about api changes and I am afraid it might break the openhab node-red package.
But just me being afraid of ghosts 
I personally prefer Rules to running a separate service to implement behaviors. But there are many who prefer to use NodeRed and seem happy with the approach.
Both would be equally as easy to switch. The Rules DSL may become deprecated but support will not be dropped.
Rules will use less system resources.
Correct. The backup script (I assume you are referring to openhab-cli backup) only backs up OH’s configs. You will need to back up the whole system or NodeRed yourself.
Thank you for your answers. I will go with Rules 