There are four different pre-order exclusives that I know of. The first is "The Naked City" which is an extra Vice Desk Case that Gamestop is giving away with pre-orders. The second is "A Slip of The Tongue", which is an extra Traffic Desk Case, and Wal-Mart is giving that away to all pre-orders.
Amazon buyers will receive "The Broderick" which is a wearable detective suit (and pistol) for you to use in-game, while Best Buy pre-orders will include "The Sharpshooter" which, like Amazon's bonus, is a wearable detective suit (and pistol) for you to use in-game.
Personally I'd go with either Amazon or Best Buy, as I'd imagine that the bonus cases are more likely to be released later as buyable DLC. That and the fact that they're probably rather short compared to the main cases in the game. At least I'd hope so, or Rockstar's going to get a lot of complaints from upset gamers not getting the extra cases.
Personally, these kind of pre-order bonuses annoy me anyway...especially if there's no future plans for releasing them to all consumers. Detective Suits, and the like...wearable items for your character, are fine, you know, whatever...but giving exclusive missions to different pre-orders is a pretty crap move.
That's why I'm hoping Rockstar will at least release these as buyable DLC's. Preferably the day the game comes out, so we can all get a taste. And I don't care what price either, I just want to play them, like everyone else. Depending on the length of the cases, I'd be fine paying 400MSP for one of the cases, and one of the detective suits, bundled together. So, 800MSP for both cases and detective suits...seems fair to me. Or if they wanted to sell them separately, maybe like 80MSP/160MSP for each of the suits, and 160MSP/240MSP for one of the cases.
EDIT: Just read an article that made me very happy. It appears that Rockstar HAS announced that all the pre-order bonuses WILL be release on the Marketplace after the game releases. Hopefully it just doesn't take as long as RDR's pre-order exclusives took to get to the marketplace (supposed to be released this month). Anyway, as long as they're released within' the month, I think I'll be happy. Definitely going to pre-order from Amazon now, because I can always do the cases later when they're released as DLC, whereas I would have to play the game completely over to get any benefit out of the detective suit if I waited to get it until it's released as DLC. So I think it'll be cool playing through the story in my detective suit, and then just going through the cases whenever they're released.
Link here: http://www.godisageek.com/2011/03/l-a-noire-pre-order-bonuses/
I feel weird about all these bonuses, especially the suits. They seem to affect the balance of gameplay by giving you an advantage you wouldn't have otherwise had. I've read the fighting is the least impressive aspect of the game, so maybe amazon's broderick suit with its alleged fighting bonus might make the game better. I don't know. I'll probably end up buying it from a retailer with no added content, which won't be a problem seeing as how rockstar games are always so massive anyways. K-Mart usually has semi-secret deals on these things, but I haven't seen any news of it on the cheapassgamer forums.
I think it's possible that the preorder bonuses will be out within a month after release as DLC... while this article doesn't state explicitly, it is encouraging (and really, unless they contracted with each retailer for a specific term of exclusivity, there's no reason they wouldn't want to strike while the iron is hot).