Never been to either, but based on my observations from this sight there is nothing that compares to Jupiter in terms of exposure. That being said all of the best players in the nation will be there so it may be harder for some kids to stand out because they are simply not as talented as some of the others.
The Underclass in Fort Myers seems like it is the precursor to the WWBA 17u tournament the following July. What I mean by that is that the 2010 WWBA underclass had pretty much all of the top 2012s in the nation and some very good 2013s, just as the WWBA 17u taking place next week will have all of the top 2012s and some very good 2013s. There will be alot of scouts at the underclass, but not as many as Jupiter and the games are spread out over many diffferent stadiums as oppossed to in Jupiter, where the whole thing takes place in the same complex. There is also a bettter chance of your players standing out at the underclass because of the lower overall talent level.