I don't know diddly about the 7509's, but I remember reading a long while back that the 7505's were more closely linked to the V500's in terms of sound reproduction (which is shit for the V500's). I've rarely, if ever, heard or seen anyone use the 7509's and Sony isn't exactly know for having a wide ranging line of good headphones; like AKG, Beyer or Grado labs.
My honest opinion is to get the V6's. They are cosmetically identical to the 7506's and sound identical too. The 7506's are marketed for the pro audio crowd and the V6's are marketed towards audio consumers. If you do pick the V6's, you can swap out the pleather ear pads for the Beyerdynamic DT250 velour pads (by ordering them through beyerdynamic). That's what I did with mine, they fit perfectly and are far more comfortable.