Thanks very much everyone for your input! I think, then, it's a toss-up between Лермонтов and Булгаков. Герой нашего времени sounds quite interesting and promises that brilliant Russian philosophical writing I crave from Tolstoy and Dostoevsky. I was also looking at Белая гвардия by Булгаков, to which I'm attracted because of its historical and autobiographical nature.
I think I will just spend some time reading a bit of each and see which will be more useful/educational for me at my current reading level.