Jake - I don't think anyone outside the admissions boards knows for sure what WP gives and doesn't give credit for, but it's one more thing that looks good to them and to the MOC's, so I'd certainly list it. Is it worth more than say French club treasurer or a certain rank in JROTC or whatever - who knows? But it would certainly be something you could bring up at your MOC interviews - that you were selected but decided to attend SLS and learn more about the academy instead.
Similarly, if you were selected for both but opted to choose Boys' State instead, I would also still include your SLS acceptance on your MOC resumes. (WP knows you were selected for it and it shows up on the MALO database, even if you don't attend).