I didn't particulary get into any of the two, but if you like algorithmics you should check out Topcoder and http://ioinformatics.org.

Topcoder is full of highschoolers or young undergraduates. IOI is specially targeted towards them, though I think you need sponsorship from your educational institution to participate.

Faring well in any of the two is something cool to add to your CV, and is highly valued by many firms in Silicon Valley.