Is it better for writers to write couplets or calligraphers to write couplets?

Not necessarily. Calligraphy is not only a technology, but also an art, which expresses the writer's soul and emotion. On the technical level, calligraphers are naturally superior, but on the artistic level, writers, as creators of couplets, can better integrate their feelings into their writing, sometimes breaking through the bottleneck of technical level and giving people a stronger visual impact.

One more thing you have overlooked is that calligraphy, as an art, is inseparable from culture, because many calligraphers in the history of China are writers, such as Wang Xizhi and Yan Zhenqing, and many other writers are also calligraphers, such as Su Shi, a great writer, whose calligraphy is also a must, and Li Bai's cursive script is also quite brilliant. So, if you ask this question about the ancient situation, it is that even the question itself has shortcomings.

Of course, it's still far from there. Now literature and calligraphy are out of touch, and tradition is lost. If you are asking about the current situation, you should pay attention to the content. Writers should write well and calligraphers should write well.