I'm going to buy the DVD, just to get the USS Constitution from Play.com.
Well, it doesn't cost anything extra.
Since the game is fully tied into Steam, it's possible that you can just enter the CD code from the box and download it anyway if you want, so it'll most likely be tied to your account anyway, there are already a few games where something like that seems to be possible, it should certainly work with Valve games you buy in a box.
Buying it from Play.com just gives me a lot of exclusive units for the same price the Steam download without any exclusive units would cost, easy choice I say.