Hi dodo
That is very difficult to decide based on the simple information.
Over the next 6 months you will earn an €4k less by taking the permanent job.
How employable are you generally? Will you be able to walk into another job on completion of the 6 month contract?
How "permanent" is the permanent job? Is the company financially healthy?
I would say if you like the lower paid job, if it's in a good company, and generally if it's good experience, take the permanent job.
Brendan