I have used both to learn Spanish for about eight months, spending on average 15 minutes a day with each. I just completed the Duolingo “language tree” (at level 14), and I am through 14 % of Babbel (I’ve completed 139 of the 991 lessons that is available in its Spanish course). The number of words/phrases mastered as reported by Duolingo is 1588. The number of words/phrases mastered as reported by Babbel in 1035. I’ve also tried out a number of other free and premium online language learning resources, but Duolingo (free) and Babbel (premium) are the only ones I’ve used beyond testing.
First: Yes, I like Duolingo a lot. I like that is is free, and I am entertained by the quizzes and gamification of Duolingo. And I also like that you can click on the “discussion” icon for almost every quiz and read comments by other learners, some of them native speakers or advanced students with a much better grasp of Spanish than me. This really helps me understand tricky stuff like the subjunctive and indirect pronouns. This is a feature missing in Babbel. Duolingo also has (very lenient?) speech recognition that at least is fun to use. Babbel’s speech recognition clearly thinks I suck at speaking Spanish (which may be true), so I’ve stopped using it.
However, Duolingo also has some shortcomings: The number of words and phrases in Duolingo at my point of “completion” was 1588. While I understand that Duolingo will continue to learn you new words if you carry on practicing, I don’t think it will ever take you past the A2 (“elementary”) level of the. Also Duolingo is not designed to teach you conversation. There are no dialogue, and it also tends to teach you some weird sentences (e.g.: “Tu oso bebe cerveza”). This quirk may help you memorize, but it does not help you learn sentences that may come handy in natural conversation.
If you are serious about learning a foreign language, I think Duolingo is a good way to get started, but it is not going to finish the job.
Btw., I do not think Babbel will finish the job either (according to itself, it will take you to B1 or the “immediate” level of the CEFR, so there are still 3 more levels to mastery), but I am confident that it is capable of taking me longer than Duolingo (i.e. give me a larger vocabulary, teach me about conversation, customs and culture). For the record: I do not think any computer program is capable of taking you all the way to mastery - at some point, you must start using the language in its real environment to progress further - but I am probably two years away from the the stage were learning a foreign language from a computer program offers no further benefit.
Babbel uses a much more conventional approach to teaching a foreign language than Duolingo. There is no gamification, and very little to motivate you (you get a “Diploma” after completing a course, but that is about it). I must admit I that I have resorted to creating my own progress meter in MS Excel to keep some track of my progress and to keep myself motivated.
However, Babbel actually tries to teach conversation. It has segments that focuses on specific situations and helps you learn phrases that will be useful for tourists and business travelers. Unlike Duolingo, there is no “unlocking” of segments. If you (like me) are a total beginner, there are a sequence of paced courses that will take you from “beginner” (A1) to “immediate” (B1). If you want to learn specifically about ordering in a restaurant, or to talk with Spanish friends about family and relationships, etc. you can go through topical segments on those (and many other) subjects at any time.
Babbel also has specific segements that are all about pronunciation of specific sounds, or to teach you about the rather quirky Spanish grammar. Those segments are quite boring, but I think that unfortunately, you need to go through such materials to learn Spanish as an adult (immersion only works well for kids).
Thanks to Duolingo and Babbel, I am already able to order at restaurants, to shop for groceries, and ask for directions when I visit Spain. At my current pace, it will probably take me two years to get through all the 991 lessons of Babbel, and by then I expect that I will also be able to engage in (simple) conversations. For me Babbel is worth the monthly fee.
To conclude. By all means get involved with Duolingo. It is free and it is fun. But if you want to learn enough to actually use the foreign language in conversation, you need more than what the Duolingo quizzes provides. For me, Babbel is working well for that purpose.