Open or Closed back Headphones

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I am considering either the Sennheisser HD650's or the 280's. The 650's are first choice but can anyone tell just how much music (noise to the wife) escapes and how intrusive it is likely to be and how much external noise may get into them? The second choice 280's stop a lot of sound both ways apparently but I think for longer listening periods the 650's would be more comfortable, cooler in summer ect.