All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Encouraging the recitation and memorization of the Book of Allah, The Exalted, is established by the texts of the Sharee‘ah. It is narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever reads a letter from the Book of Allah will get a reward, and the reward is multiplied tenfold. I do not say that 'Alif Laam Meem' is a letter, but 'Alif' is a letter, 'Laam' is a letter, and 'Meem' is a letter." [At-Tirmithi, Al-Albani: Saheeh]
Also, Abu Hurayrah narrated that the Messenger of Allah, sallallaahu ‘alayhi wa sallam, said: “Allah has ninety-nine Names, one hundred minus one; whoever memorizes them, calls to Him using these Names, and acts according to them will enter Paradise.” [Al-Bukhari and Muslim]
In his commentary on Saheeh Muslim, An-Nawawi said: “As for the Prophet's saying: ‘whoever memorizes them will enter Paradise’, scholars held different views regarding the meaning of memorizing them. Al-Bukhaari and other researching scholars said that it means memorization, and this is the stronger opinion, as it was explained as such by the other version of the Hadeeth: ‘He who memorizes them.’” [End of quote]
The questioner may memorize both Surah Ya-Sin and the ninety-nine Names of Allah. This is neither impossible nor hard. It is better for you to combine both choices, for this entails a greater reward.
We cannot decisively state which of the two is better, i.e., memorizing Surah Ya-Sin or memorizing the Most Beautiful Names of Allah, as this varies according to the difference of people and circumstances. Shaykh-ul-Islam Ibn Taymiyyah said in Majmoo‘ Al-Fataawa:
"The reward, however, varies in proportion to the benefit, interest, and usefulness of the act, and in proportion to one’s compliance with the command of Allah and His Messenger. So the better of the two acts and the one whose doer is more obedient and compliant is the more excellent. In fact, the superiority of actions is not measured by their abundance; rather, the state of the hearts while doing the actions is that which decides the superiority." [End of quote]
There are many Ahaadeeth reported on the merits of Surah Ya-Sin, but the majority of them are not authentic.
"Whoever recites Surah Ya-Sin in a night, seeking thereby the pleasure of Allah, he will be forgiven." [Al-Albani: inauthentic in As-Silsilah Adh-Dha‘eefah]
"Everything has a heart, and the heart of the Quran is Ya-Sin, and whoever recites Ya-Sin, Allah will record for him that he recited the Quran ten times." [Al-Albani: inauthentic in Tahqeeq Mishkaat Al-Masaabeeh]
"Whoever recites Surah Ya-Sin one time, it is as if he recites the whole Quran twice." [Al-Albani: inauthentic in Dha‘eef Al-Jaami‘]
Allah Knows best.