Arba'een (الأربعين, romanized: al-Arba‘īn, lit. 'forty'), Chehlom (Persian: چهلم, Urdu: چہلم, "the fortieth day") is a Shia Muslim religious observance that occurs forty days after the Day of Ashura. It commemorates the martyrdom of Husayn ibn Ali, the grandson of the Islamic Prophet Muhammad, who was killed on the 10th day of the month of Muharram.
We should not conflate the reward of the Ziyaras or Hajj. Performing one does not mean we should not perform the other. Hajj has its specific times and the Ziyara of Imam Hussayn has a specific time. If you can afford to perform both, that would be great. Keep in mind, those who perform Hajj will also have the opportunity to visit RasulAllah, FatimaZahra, and the Imams in Baqee'.