Both are great, you can't go wrong with either of them. I believe you will enjoy Card College as it is easier to read and the moves are explained in much better detail.
Card College. Royal Road was published in 1948. It is dated in the way it is written and more difficult to understand. I think that Card College is the best way to learn - especially if you get a copy in German.
Yes I will! Thank you so much for your advice and for your suggestions. Without you, I probably would have bought any advanced book without knowing the fundamentals. Really nice community, I joined not a long time ago and the problem is already solved! Amazing. Have a great day!