Summary: NBA2K13 is great, MLB2K13 has so many bugs that it is almost unplayable including invisible walls in the outfield, npc's catching fly balls underneath the stands, and the player who bats after you in mycareer mode sets the record for most strikeouts in a season every year.
Summary: I really want to say that I love MLB 2K13, but it's basically MLB 2K12 with a roster update. Pretty disappointing stuff honestly, but it's the last (forseeable) baseball game on Xbox 360 so that's something to consider. NBA 2K13 is great and it's a joy to play. My career is probably my favorite mode and basketball fans should enjoy developing their player for many seasons.
Summary: I'm not much of a gamer, but I've played the NBA2K series before and love the "MyPlayer" portion of the game, where you can level a custom character through the ranks. Because of this, I thought MLB2K13 would be similarly as fun. While it's enjoyable to play for a short period of time, i feel like it requires HOURS to level up your player.