Hm, I never quite understood the reasons for this Sibelius cult. Perhaps his music is just too cold for a Brazilian to understand, it melts before arriving here. Some years ago I bought a CD with Rattle conducting Sibelius and Nielsen, this one I already knew from his piano sonata (great piece). I love Nielsen (I think it was Sinfonia espansiva) and found the Sibelius, a few months later, I found a great bargain with all his symphonies and tone poems with the same Rattle. I never got the music, found it too much rhapsodic, too slow, too monochromatic, and never went back to it. Since I arrived in this forum and found the excitement of its members, I decided to give Sibelius a go. I can't like it. I'm even listening now to his most "acessible" symphony, but for me it is just empty. Void of any meaning, even musical.
I was, as so much people are, a Mahler fan in my teens, I bought with my sparse earnings all his symphonies and songs, and had almost all of then in my head. And then I grew up, his overblown sentimentality doen't appeal anymore to me. Not that I don't like Mahler, I still listen quite frequently to most of his symphonies, and the Song of Earth is one of my favorites, but Mahler just doesn't excites me anymore. To me he is so much ahead of Sibelius, his works have a germanic coherence totally lacking in Sibelius.
So I would vote for Mahler, but that's not a great duo for me.
And as for the guy who said about the second half in 20th century music, Messiaen's Turangalîla is as good as a symphony as anything I heard: exciting, great melodies, incredible orchestration (and that Ondes Martenot). At least to me some miles ahead of everything Sibelius did and and most of what Mahler did.