This is probably too late, but I attended AP/EA last summer and am going to attend PGSS this summer. I took 15-122 and 21-127 last year. I would recommend that you take AP/EA if you really want to go to CMU because it gives them a real example of your ability taking their courses. If you want a more generally “prestigious” program, I would save the money and go with PGSS because the admissions rate for AP/EA is around 60% while PGSS is ~10%.