Is fard kaza salat more important than sunnah salat.
Certainly: fard is a due on you while sunnah is optional. By simple logic if you had a duty and missed to do it, would the person who issued it accept it and reward you (the same) if you've made something else?
Further in a qudsi hadith Allah has said:
Allah's Messenger (ﷺ) said, "Allah said, 'I will declare war against him who shows hostility to a pious worshipper of Mine. And the most beloved things with which My slave comes nearer to Me, is what I have enjoined upon him; and My slave keeps on coming closer to Me through performing Nawafil (praying or doing extra deeds besides what is obligatory) till I love him, so I become his sense of hearing with which he hears, and his sense of sight with which he sees, and his hand with which he grips, and his leg with which he walks; and if he asks Me, I will give him, and if he asks My protection (Refuge), I will protect him; (i.e. give him My Refuge) and I do not hesitate to do anything as I hesitate to take the soul of the believer, for he hates death, and I hate to disappoint him."
So even if some scholars say that a missed prayer can't be performed as qada', performing sunnah prayers instead isn't equal (even if there are scholars who consider this hadith as an evidence for praying nafl instead of qada'). Certainly performing a prayer as qada' also isn't the same as performing it on time, as one has missed all the rewards especially if this delaying was intentional.