Is it preferable to prolong the recitation with long surahs during prayer, or is praying with short surahs a good thing, knowing that parts of the Quran have been memorized?

It is not obligatory for the worshipper to recite short or long surahs. Rather, it is sunnah to recite something from the Qur'an along with Al-Fatihah, and it is recommended that it be a complete surah or a few verses. It is recommended to lengthen the recitation in Fajr prayer, then Dhuhr, then Isha. This is a recommendation, not an obligation. As for the Imam, he should make it easy for the congregants, while an individual worshipper may lengthen their recitation as much as they wish.