Duolingo is the fun, free app for learning 40+ languages through quick, bite-sized lessons. Practice speaking, reading, listening, and writing to build your vocabulary and grammar skills.
Having used both programs, I can say that Babbel is better than Duolingo in terms of effectiveness, engagement and offering guidance. I like that Duolingo is free, but that's about it. Babbel is hands down the better overall language learning app.