Yeah some headphones are more open than others in terms of sound leakage and environment coming in.
Even though the HD650 back is completely open, the AD900 has much much more airy sound. And the AD900 leaks a ton more and can hear just about any external noise. I don't know why the HD650 sounds different and seems to isolate better, but it does.
Headphones like AD900 are just incredible for that air sound!
As John also said, don't let Open Air fool you. The HD650 has the best bass I've heard from any headphone so far, closed or open. While the AD900s female vocals are ethereal, just musical bliss. I don't like switching from AD900 to HD650 and back. I decide what I want to listen to and what headphone I want to listen to. Because listening to Beethoven on AD900s sound completely different from the HD650.
So good luck, you just have to hear for yourself. Don't let yourself be contained by the terms. Open Air does not mean the AD2000 will sound like the AD900.
Jaren - Let Me Breath for example sounds infinitely better on the AD900s IMO.
Beethoven's Symphony No. 3 sounds better on the HD650s though.
Also: Open Air headphones are good for conversations. I am probably the only person I know person to person that owns an Open Air. And when friends first heard me listening to music on them, they all were startled thinking music was too loud and I must be destroying my ears. Then they heard for themselves and realized Open Air, it's truly Open Air, sound goes in and out. And since listening to my AD900s about 3 years ago, well I don't listen to closed unless I absolutely have to!
The other part you will discover, COMFORT. The AD900s are more comfortable than any Closed Headphone I have tried yet. They are like wearing air. The HD650 had some clamp, but when it loosens up, also very comfortable. In general I think Open Air are more comfy and less sweaty ears, well I never have sweaty ears with either HD650 or AD900.