Two team have quite big differences and we will never be able to equalize them, moreover Al-Qadsiah development will really be tested in the SPL competition next season, they will be under series of pressures against strong SPL team who always dominate.
If we compare where those teams finished in every season, Al Ahli is surely one level above Al-Qadsiah. Al Ahli mostly finished the season in the top 3, they even became the champion several times. Also, Al Ahli never relegated to second tier league but Al-Qadsiah experienced it last season. So simply Al Ahli is much better than Al-Qadsiah, Al Ahli is one of the top teams in Saudi league. And if we compare their trophies, it is more visible that Al Ahli is not equal with Al-Qadsiah.
References:1.
https://www.transfermarkt.com/al-qadisiyah-fc/platzierungen/verein/260692.
https://www.transfermarkt.com/al-qadsiah-fc/erfolge/verein/260693.
https://www.transfermarkt.com/al-ahli-dschidda/platzierungen/verein/184874.
https://www.transfermarkt.com/al-ahli-sfc/erfolge/verein/18487Regarding Nacho, it is clear that money is his main goal, it is impossible for him to play with Saudi team just for low salary, but this is Saudi football which can always bring in lots of European players.
Besides money, Nacho wants to play regularly. If he stays in Real Madrid, he must compete with other defenders for the same position. It will be harder due to his age, other defenders in Real Madrid also has a better quality. So, it makes sense if he leaves Real Madrid and joins Saudi league. In this league Nacho gets a lot of chance to play and he may become the captain in Al-Qadsiah.