But in addition to that, I will agree with you that stylistically it's a very different book. My understanding is that he first wrote the short story version of Ender's Game, then years later had the idea for Speaker for the Dead. He realized that Ender could be the one to carry the torch, so he rewrote Ender's Game as a full-length novel in order to set it up for its very different sequel. Both won Hugo and Nebula Awards in consecutive years, which also interests me. One is very internal to Ender and quite action-oriented, the other more character-driven and tied to complex moral dilemmas. Ender is a very different person after years of trying to repent for his transgressions, however inadvertent they were. I really did enjoy Speaker, though it is such a departure from the first.
As for other series, I haven't read anything else, though Card is quite the prolific writer. His Earthfall series looks really interesting to me...it's supposed to be a science-fiction retelling of the Book of Mormon. Sounds fascinating to me, and I hear it had some good reviews. I'll get around to it eventually.