Why is there a distinction like the Spiral Path and Direct Path? Don't they both lead to salvation?
Yes, but the results and the length of time required in each are different.
The spiral path leads towards the Absolute throughout many eternities, many cosmic days or Mahakalpas. Nirvanis incarnate and work on the mind little by little. They receive wisdom through conquering small ordeals. Since their trials are fewer, their victories are smaller. This is like developing the light of a candle.
However, the amount of light generated by annihilating the ego in one life is like liberating energy within the atom. It is truly revolutionary, because through the amount of work involved, it takes little time and produces more lasting results.
Nirvanis often fall because their incarnate in this world again and are tempted. Bodhisattvas, while risking everything, including their life, achieve resurrection and are in a better position to not fall again, although this is no guarantee.
The Qur'an speaks about the djinns or jinns and human beings. These are the adherents of the spiral path who do not follow or believe in the signs of Allah. 'So which of these signs do you two deny?' states Surah al-Rahman, the Compassionate.
Only those who are guided on the straight path are those who are bestowed the most divine favors, since it is the path of those who do not incur wrath, nor of those who go astray. See the first Surah, al-Fatihah, the Opening.