It's interesting to think about.
If Gore was president on 9/11, at the very least I think it's safe to assume we would not have invaded Iraq, so you might have to guess what the trade-off would be to have Saddam Hussein potentially in power for 20 more years compared to the rise of ISIS (gross oversimplification, I know). You might also wonder if the GFC in 2008 or climate change would have had different trajectories. I can also see a scenario where 9/11 becoming the galvanizing point of GOP obstructionism rather than Obama's election, leading into something near our current state.
I can't imagine a GWB presidency without 9/11. He was basically vacationing from inauguration up to that point. It probably would have been a bland neocon term, and his reelection would have been more competitive. Maybe the Dixie Chicks would have had a better career.