pabaseballdad,
Years ago I would tell you that Jupiter would be the easy choice. However things have really changed with college recruiting. At one time very few recruiters would attend the underclass events. Now they come in big numbers, including many of the top college programs in the country. The landscape has changed and colleges more and more are recruiting underclassmen.
Perhaps the biggest difference between the two is the number of MLB scouts is astounding at the Jupiter event and not nearly big many at the underclass. The underclass event is not quite as high profile as Jupiter.
Some depends on just how talented the player is. If he is a potential early draft and already made a college commitment you will be happy with both, but obviously Jupiter would be better.
Actually BucsFan suggestion makes sense. Even though it might not be necessary or even the best choice... I just wish every good player could experience Jupiter before they graduate from high school. It's probably something they will never forget no matter where their baseball career takes them. Nothing fancy about it, it is as grass roots as high level baseball can get. Just much different than anything else I have experienced in baseball.