After a lot of consideration, I just ended up going with the bare bones version of Dark Knight as well (the $15 one). I usually spend the extra to get the "deluxe version" of movies like this (Star Wars, Indy, Spidey, etc.) - but looking over the extras, and being a little tighter with the budget lately, made me decide that the extras weren't worth the extra $8 (at Target). When it came down to it, I just wanted the free digital download, but when I really thought about it - I don't really need that either. I always try to pick up the versions that have that, thinking that "someday" we'll get a newer iPod that plays video - but I know that's a ways off, and even then, how often am I going to look at that tiny screen to watch a movie as long as the Dark Knight. If I want to watch it on the computer or a portable player, I can just use the DVD. Plus, eventually I'd like to upgrade to blu-ray (if/when we can afford it), so I thought the re-buy wouldn't sting quite as badly if I got the cheaper version now.
As a side note, the Dark Knight was selling like crazy at Target last night when I picked up our copy (and one for my Dad for Christmas). It seems like most people were grabbing the single disc, which I suppose appeals more to casual buyers anyways.