I received the exact same letter on the same day. I am also in the same boat with not knowing whether to accept or not because I have already been offered a full ride to a number of schools. This is a major life decision to make, and I have less than a week to decide how I will live my life... no pressure.
If the academy is what you want, take the FF Scholarship. As long as you don't screw up academically or have discipline trouble, you'll have your appointment for the C/O '22 in hand by March. I think my prep year at MMI has better prepared me for the Academy, both academically and mentally. I got a little taste for the military lifestyle while also making friends that I'll surely stay in touch with at the Academy and the rest of my life. Any of the military prep schools will provide you with just a hint of Academy life to help you make sure your decision is right.
Additionally, you still take AROTC (all sponsored cadets take Army ROTC) but there have been instances where sponsored cadets have been authorized to go to the University of Alabama with the self-preps for AFROTC. If in the end you decided that the academy isn't what you want, after paying back the Falcon foundation for their contributions, your credits from MMI will transfer to many other colleges and you can reapply for an AFROTC scholarship if you so wish.
I was offered an AFROTC scholarship last year as well and I couldn't be happier with the decision I made to pursue the academy. Good luck and if you have any questions, feel free to DM me!