The first procedure gets you up fast - a lot of noise at a local area for some time.
The other gets you away fast - a lot of noise but distrubuted over a larger area.
Which procedure that is used depends on the surroundings - correct me if I'm wrong.
And a little joke:
"Flight 103, descend FL290"
"Roger, leaving 330 for 290 ... what's the reason for that, Sir ??"
"Noise abatement"
"Noise abatement ??? We're cruising at high level Sir, what kind of noise abatement ???"
"I know, I know. But there's opposite traffic at level 330 ... and there will be a LOT of noise if you both meet at FL330 !!!"
[This message has been edited by Tor (edited 11 April 2001).]