They are identical. There is no DRM in any of the games, so once installed, you will not need to keep the DVD in, or have the Steam client running. There is a launcher for the DVD version of the games, which will check for and download/install a couple small updates for DD and BD.
If you don't care about Steam achievements for D2:DC (or playtime tracking), then I'd say install the DVD versions of the games, and activate the Steam codes in case you loose the DVD or want to install the games on a new computer when the DVD isn't handy.
