The levels are parallel but it’s easier to get promoted to level 6+ on the management track.
but, I think the answer depends. If you are a really strong individual contributor, I presume getting an IC promotion is easier. If you are just decent, getting an IC7+ promotion seems incredibly challenging because IC7 requires an very high level of competence. If you are just decent as a manager, you can get luckier.
M1 corresponds to the same pay band as IC6 and M2 corresponds to IC7. D1 corresponds to IC8. (By level 6, I am referring to IC6 / M1.)