Really Sfar? They're all 1/4" where I live. :) Perhaps they can be ordered with either, that would explain things!
In any case, the three phones can all use the same cables since they have the same connectors. The 600s sound slightly better with the 650 stock cable, so that could be a worthwhile upgrade. However I still recommend getting something better, there's plenty of good ones out there. :)
In any case, the three phones can all use the same cables since they have the same connectors. The 600s sound slightly better with the 650 stock cable, so that could be a worthwhile upgrade. However I still recommend getting something better, there's plenty of good ones out there. :)