What advantages does Duolingo offer over Rosetta Stone?
First, Duolingo is totally free. No ads, no subscriptions, no 5-easy-payments plan. Rosetta Stone costs hundreds of dollars.
Second, there is scientific evidence that Duolingo can be more effective. An independent study was done by a research team that previously evaluated the effectiveness of Rosetta Stone. They found that it takes students on Duolingo 34 hours to learn the equivalent of a one-semester university course, whereas it takes 55 hours with Rosetta Stone. Here is a link to the study: http://static.duolingo.com/s3/DuolingoReport_Final.pdf
And of course there are other differences that are harder to quantify: Duolingo has game mechanics, a 5-star mobile app, and since it doesn't come in a CD-ROM, we're able to improve it on a daily basis.