Listened to six or seven different versions Mahler 6 and thought Berstein's was the best. In the 60s some conductors cut out the exposition. To me, this feels unacceptable, but I guess it was considered "optional" at the time. Szell has a very good recording without the exposition.
In the early Romantic era, it was common practice for composers to write in the repeat without having any expectation that it would actually be done. The repeat was a staple in Classical era Sonata Form, not so much post-Beethoven. That's mostly what surprised me when listening to Mahler's 6th. Perhaps, when the time period that Mahler was alive is considered, it could be seen as tribute to a forgone tradition. But yeah, if you listen to any of Schumann's or Schubert's symphonies or chamber compositions, the repeat is almost always written in even though it's rarely done.
Never really listened to Schoenburg; where do you suggest starting?
Verklarte Nacht is probably the best start. It was written before Schoenberg devised his twelve-tone system, so, though it is dissonant and modulates rapidly, it is tonal. It's my favorite work by him. I'm not sure if I would recommend the original string sextet format or his later transcribed string orchestra version. They're both great, so I would just go with whichever instrumentation you prefer. Probably the most comparable work that came before it was Brahm's Piano Quintet.
His
Gurrelieder is a great tonal work by him as well, but only first part was written while he was still "young."
If you want to delve into his twelve-tone works, I would suggest his
Variationen fur Orchester. For whatever reason, music history courses always use
Perrot Lunaire, but I believe it was composed when Schoenberg turned atonal, but it was before he put the finishing touches on his twelve-tone system. Thus, it doesn't reflect his other works very well, other than the use of
sprechstimme. But there are better works to hear that, such as "Ode to Napoleon Bonaparte" or "Survivor from Warsaw." I've gotten off track, but
Variationen fur Orchester is definitely the best start for his twelve-tone works. It's fairly brief and very hard hitting. One listen proves any critic wrong who claims that, just by music being atonal, it cannot be called music.
Also, if you're a big fan of Mahler, I would suggest that you check out Alexander Zemlinsky. Specifically, his Lyric Symphony and Little Mermaid. He's very similar, though not quite as conservative. Interestingly, he was in a relationship with Alma Schindler before she dumped him and married Mahler. He was also Schoenberg's only formal teacher, in which he taught him counter-point.
Hey Onder, have you listened to Arthur Honegger?