ה,טז כל מחלוקת שהיא לשם שמיים, סופה להתקיים; ושאינה לשם שמיים, אין סופה להתקיים. איזו היא מחלוקת שהיא לשם שמיים, זו מחלוקת הלל ושמאי; ושאינה לשם שמיים, זו מחלוקת קורח ועדתו.
This line of Pirkei Avot differentiates between worthy disagreements, such as that of Beit Hillel and Beit Shammai, and unworthy disagreements such as that of Korach and his party.
It seems that we can further refine this distinction by looking carefully at Korach and his party. Essentially, their rebellion had two distinct foci: Korach, who wanted to be a priest; and Datan and Aviram, who wanted a political leadership that would not strand them in the desert for 40 years. Korach's goal was to more meaningfully (so he thought) serve God, while Datan and Aviram's complaint was purely material. It seems that Korach's basic motivation was praiseworthy. In contrast, there was no such redeeming quality to Datan and Aviram's complaint.
It appears that this moral difference is reflected in the punishment received by each party. Datan and Aviram were swallowed up by the earth (see verse 16:27), their bodies and memory erased from sight forever. Korach, in contrast, was burned to death while offering incense with 250 of his followers (see 17:5, also see 16:27 which makes a point of not mentioning Korach). The incense shovels which Korach and the others used were beaten into a covering for the altar. This memorial to their actions has a negative purpose, as a warning. But at the same time it functions as an integral and positive part of the Mishkan, which makes me think the lesson to be derived from it is not entirely negative.
Both halves of the rebellion were unjustified in that they involved knowingly disobeying Divine commands. Thus, Pirkei Avot is justified in lumping them together as a single unjustified argument destined not to endure. But while Datan and Aviram had entirely base motivations, Korach's motivation was praiseworthy in its source. Thus while the legacy of Datan and Aviram vanishes completely, Korach's legacy does not, and a memorial to his motivations is preserved forever in the Mishkan.