Which is better during the daytime in Ramadan: reading the Quran or performing voluntary prayers?

It was part of his guidance, peace be upon him, in Ramadan to increase in acts of worship, especially reciting the Quran, giving charity, doing good, praying, performing dhikr, and observing i'tikaf. As for preferring Quran recitation over supererogatory prayers, it varies according to people's circumstances. The best deed is that which brings its doer closer to Allah the most.