If Aunt Julia and The Scriptwriter represent Mario’s two loves (romantic love, literature), then what does the book seem to say is more important to happiness?

In my opinion, I don’t think the book is portraying one love as more important to the other – it is saying that both loves contribute a great amount to his happiness. Sometimes his love for aunt Julia may be more important based on his situation and Vice-Versa, but overall they both matter and amount to his happiness. His love for Aunt Julia is romantic, its something thta brings joy to his life not only when he is with her but also when he thinks about her and their relationship. His love for literature is more like his passion for his job, the thing that motivates him to wake up everyday, go to work and try to make a difference. Both these loves contribute to different aspects of his happiness, but they both equally contribute to his personal happiness.