Depends on the game, but I voted for the content.
I don't care much for soundtracks, and while looking through the concept art for some games is quite fun most of it usually ends up on the wiki for the game, especially if it's a larger game that gets its own (Dragon Age, for example). However, things like figurines, special boxes and other nifty trinkets can sometimes make me buy a special edition of a game. I didn't get the Dragon Age CE because it contained nothing interesting, but the ME2 CE came with so many nice things I couldn't pass it up, and I don't regret buying it for a second.
Though as I said it depends on the game. If it's something large and possibly groundbreaking I've been waiting for for a long time, I'll get a special edition. I didn't think I'd actually end up loving Fallout 3 so I didn't shell out the extra 5 (oh yes, five) euros for the CE. Man I hated myself when I saw what it contained.
In short, it's a split between content and the game in question.