Which is better and more rewarding in the sight of Allah: praying two rak'ahs of Qiyam Al-Layl with two juz' (parts) of the Qur'an, or increasing the number of rak'ahs with the same two juz'?

If the standing (qiyam) is equal in a two-rak'ah prayer and a four-rak'ah prayer, then the four-rak'ah prayer is better due to the increase in bowing (ruku') and prostration (sujud). However, if the standing in the two-rak'ah prayer is longer, then the two-rak'ah prayer is better according to some scholars, based on the Prophet's (peace be upon him) saying: "The best of prayers is that with prolonged standing (qunut)." And if the standing in a four-rak'ah prayer is longer, then it is better a fortiori. Lengthening the standing is better than increasing the number of rak'ahs, according to An-Nawawi.