Just ran into this question… You may have gotten the answer already. But here are my two cents…I had never heard about Horizon Academic before, so I checked out Horizon’s website and really wonder how legitimate it is. I am an IEC and a mother of two high school students, so I am very familiar with the quality of different summer programs. Horizon seems to be run by a small company that focuses on padding the CVs of rich Asian kids by connecting them to professors from big name universities.
You need to be very careful with what programs you associate your application with given the rise of recent college admission scandals.
I also heard Pioneer is quite respected. Good luck!