Central and South America each offer unique attractions and both offer the chance to learn and practice Spanish. South America has more famous sites (Machu Picchu, Nazca Lines, Salar de Uyuni) but a visit can make you feel like a tourist cog in a money-making machine.
Central America has great attractions too: Tikal, for ruins, the Panama Canal for marvelous feats of engineering, Antigua for Colonial towns, the Corn Islands for beaches, Costa Rica in general to be astounded by nature.
The difference is, in Central America, you're far more likely to experience your destination outside the tourist whirl. You’ll often be the only foreigner on the bus or strolling around town. This won’t be the case in Antigua, Guatemala, or Roatan, Honduras, but if you visit, say, Tegucigalpa or Panama City, you can wander the streets feeling a real sense of the city without being pestered to buy finger puppets or tour guide services.
And because Central America is much smaller than South America, you can spread your trip around two or three countries and still get a good taste of each. If you want a less touristy experience but still want to stay on the beaten path, try Guatemala or Costa Rica. If you want to check out places that have not quite hit the tipping point on foreigners’ radar, Honduras, El Salvador, and Nicaragua all offer unique experiences.
Of course it's all a matter of personal taste, and you certainly won't go wrong visiting South America. And as Feature Writer for Latin American and Caribbean Travel, I’m clearly biased. But having traveled extensively in both Central and South America, I’d definitely pick Central America to go back to.