Back with Mahler6. Form can be annoyingly labyrinthine, directionless, orchestration always beyond reproach. Sibelius is the exact opposite.
Think of Beethoven 1st: all proportions perfect. In Mahler the poetry of geometry is replaced by a narrative. A powerful one for sure.
Sibelius had a gift for organism-like musical forms. His orchestral technique was never completely reliable Most of my colleagues agree.
[I am a fan of Salonen's Sibelius; would pay folding money to hear him conduct Mahler....or anyone else, for that matter.]