It's sad now as it is forgotten, but I think the bombing of pearl harbor is more significant. For one, it was the first time another country intentionally attacked us with the intent of war and crippling our capacity for it. For two, the wttc bombing is still up for conspiracy and no country has claimed fault, meaning it's tough to signify what the wtc bombing purpose was and if it was accomplished. And for three, pearl harbor prove the American empire could rally together in the time of need, and since ww2, we have not rallied ever since, to that extent anyways.
This was a greater paradigm shift than 9/11. The entire world was upended by this.