Sealed is better, because it's way stronger than unsealed. Unsealed bearings get destroyed after a bad crash but sealed bearing continue to live and they will probably outlive yourself.
Sealed headsets are way easier to install and uninstall. Less maintaining needed.
The thing that people say that unsealed spins faster is just rubbish, because you most probably don't need to do 100x barspin within one air and one throw. People who say that unsealed spins way faster probably don't know anything about compressing forks and headsets to a certain point where the scooter is solid and at the same time it spins well, regardless if they have a sealed or unsealed headset. You can still do your barspins as many as you want.
I've ridden sealed headsets on my scooters since October 05 and i never want to use an unsealed again. I've been using my last headset for over 15 months in a row and all my previous sealed headsets are still working fine today after being sold to BMX bikers.
Sealed headsets are the future and well worth investment.