Two of the greatest symphonists of the 20th century...but who is greater?
Sibelius and Mahler both took on the symphony with quite different philosophies. In their famous exchange, Sibelius said: " I admire the symphony's style and severity of form, as well as the profound logic creating an inner connection among all of the motives," wheras Mahler said: "The symphony is like the world; it must embrace everything."
Who is right here? Both? Neither?
As an admirer of both symphonists, my vote goes to Sibelius. While Sibelius's seven symphonies often lack a sort of "hysteria" and hyper-emotion that one encouters in Mahler, his works can still certainly elicit strong emotional responses. And he does this within fairly strict means, concentrating the musical rhetoric so every theme, phrase, motive and note seems to be concentrated with meaning.
Plus, Sibelius seems to have a masterful handle on the symphonic form, which I think is important here. A symphony is not a suite or a rhapsody; it, by its very definition, has rules and conventions. Sibelius seems to take the symphony head on and make music that adheres to the "severity of style." whereas Mahler seems to go more rhapsodic and bend the rules quite a bit more.
Not that there is anything intrinsically wrong with that; again, I love Mahler's symphonies. But from a technical standpoint, Sibelius seems to understand symphonic form much better.
Obviously, there are no right or wrong answers here; not one of us can say definitively who is the greater. But I think a civil and respectful discussion on this would be most interesting!