If I were to buy this game in ONE format: Steam or iPad - which would you suggest and why?
Which is the best version of this game?
Since they are pretty much the same, it is just up to your preferences. If you want to play it on your PC, go for Steam version, and the same goes for iPad.
There are 2 formats and there are different in many ways...
Steam/IOS/Android: "ELDER SIGN: OMENS" this has lots of videos and music that makes the game more dark and immersive, plus having different campaigns adding some sort of story flavor. After you defeat/lose any Boss you get a video, making it like a closure that the board game lacks.
Board Game: "ELDER SIGN" Very different from the virtual version, all what previous post says is true.