Is there any benefit to buying the box version of Civlization 5 on Steam versus the box, or vice versa?
All Civilization V version are basically the same because at the end all have to be registered on Steam, because it implements the Steam DRM.
Then could be that some reseller add some exclusive stuff:
Steam Standard edition
Steam Deluxe edition
A boxed collector's edition is also coming. It should contains some miniatures, a book and possible other contents like soundtrack/make of.
The price should be around 100$.
The box version loads itself into Steam, so after you install, there is no difference between the two on your computer.
The boxed version includes a tech tree poster. There is an important typo on it though. It lists Archery as a dead end technology, which it isn't. The "manual" is a two page flier on how to install the game.
Wherever you source the game, it makes little difference. The game uses Steamworks, so it will be linked into Steam anyway.
With pre-orders from on-line retailers, you get the game at the same time (though you have to spend 15mins installing it), but you get it cheaper. In the UK, you are looking at £25 vs Steam's £30.