Adventure Time is a show that probably better explores complex ideas and gets into adult themes and concepts without alienating young viewers. Steven Universe is made with higher production value, so it looks, sounds, and feels more polished as a show. Which show is better really depends on your tastes.
People who enjoy Adventure Time watch it because it has clever humor, dark undertones, and finds approachable ways to explore heavy topics. People who dislike it often are turned off by humor aimed at kids and an odd art style. Whether or not it’s worth watching really depends on your specific tastes.