Brilliant that your DC is taking two language GCSEs and with such high PGs - excellent!
As others say, Spanish is spoken by more people worldwide, but German is said to be more sought-after by industries (such as engineering).
German has the advantage of being much less common now and thus may have the appeal of rarity. But both are equally respected by unis.
I agree, look at the spec and ask at school which book and film will be studied. I love love Leben des Anderen and Goodbye Lenin! Those would be the likely choices for the film, along with Der Besuch (amazing play) or Der Vorleser (also good) for the book. Other options are available but be aware that those are the only ones with an indispensible revision guide. (I'm talking AQA here btw). (One book and one filme btw, not two!)
I don't know the Spanish spec (Germanist and French too here) but it will be more than possible to watch the films and check out the subjects and themes of the books.
Also they will have to do an individual research project on something relating to the language and be able to speak about it for up to 12 minutes - so is there anything in either language that catches their eye? I am thinking, are they a massive fan of Bayern München or biiiiig into Pedro Almodovar or Picasso?
You may also want to consider that in years gone by the grade boundaries for German have been considerably lower than those for French, and to a lesser degree, Spanish. As an example, an A grade at AQA A level Spanish last year required a mark of 307, while in German 292 was needed. The exams are identical in structure and form. The disparity is even greater for French (A was 320 in 2023).
HTH please ask anything else tho.