I completed a PDC in July in Bega (Sth NSW). I looked into doing one via correspondence but in the end chose not to.
The major disadvantage that you encounter by doing it online is you miss the opportunity to meet and discuss your thoughts with other people (fellow students).
Apart from being a very good course, one of the best things that I got out of the PDC was this interaction with others. I met some truly inspiring and intelligent people on that course (turors and students). If I had done it by correspondence I would have missed out on that.
You should never cry over spilt milk,
you can always wipe it up with a sponge & squeeze it out over the childrens cereal....