This is almost apples and oranges. Mahout's power lies in it's ability to handle huge amounts of data, in a parallel fashion. Weka (which is rarely used these days anyways) is for smaller problems and experimentation.<p>None of these (Mahout and Weka) are mainstream anymore. For large-scale classification, people are using packages like VW[1] . And for small-scale experimentation, SciKit or R.<p>[1] <a href="http://hunch.net/~vw" rel="nofollow">http://hunch.net/~vw</a>