I have to brag up a bit. I was the Coast Guard Academy faculty advisor since 2006, and I have a number of alumni active duty and presently at the academy. At my last count over 10% of the CGA enrollment are either MMI CGAS or MMI self-prep including the two students who were selected for graduate school in this year's graduating class.
There is no heirarchy for prep schools; no one school is better than another. Each offers their own unique experiences. Each school will well prepare a student for success at the academy using the programs that are established.
What makes a school hard or easy is the student. Study habits, interaction with instructors, seeking out help from peers and instructors, asking questions in class; these are all things that will help a student succeed, regardless of what prep they attend.
My first day in ROTC the unit CO gave a short welcome speech. He said, "Throughout your career, you will go to your assigned station, and you'll do your assigned tasks to the best of your ability. That's how you're going to be measured." Prep school is your first opportunity to carry this out. You will have a lot of support regardless of where you go, and you'll be given all the tools you'll need to succeed.
Go forth and do great things!