I feel bad that no one has replied so I'm going to attempt to, even though I am not familiar first hand with the exact diplomas that you have mentioned.
In Canada, after having completed your last piano grade, you basically can take either a teacher's of a performer's ARCT. In England, there is a similar choice, depending on the college of your choice. I did four years at the Royal College of Music and then took the teacher's ARCM.
The general feeling was that the teacher's exam was more intense, with more theory and 'away from the keyboard' study. The performer's version was mainly concerned with your practical level of playing. Most of my colleagues who were interested in a well rounded education opted for the teacher's exam.
I hope you can relate some of this to your circumstance. Best of luck.