Will the GOG version contain all bonus content available or will other vendors have exclusive content if I pre-order from them that I won't get with the GOG preorder?

I think you get all bonus content available elsewhere if you get the gog version, and in adittion you'll get exclusive content that won't be available anywhere else, not to mention the free game gift ;)

You will get all the extras you get with the Premium Edition in a digital format (so manual, map, etc in documents form) plus a digital version of the CE Artbook plus the choice of a free game from GOG. You won't of course get the other stuff exclusive to the CE version, but out of the digital distributors I think GOG is your best bet. The stores selling hardcopy versions of the game are offering exclusive dlc like the Roche commando jacket but I haven't seen anything equivalent from the digital suppliers.

GOG's version is also DRM-free. However, I do know that Steam seems to be offering all the digital edition premium stuff too (though I do not see any mention of the digital artbook with that version, I think that is exclusive to GOG), however they are not offering the free game and you may find it harder to cancel the preorder from them (though it can be done, as I mysellf have managed it).

Yeah, my answer was just in regards to digital distributors, retail is a different ball game.

Oh dear, you might be interested in this...
The Witcher 2 Premium & Collector's Editions Detailed (Plus Pre-Order DLC)
http://www.incgamers.com/News/26028/the-witcher-2-premium--collectors-editions-detailed-plus-pre-order-dlc