steam is a very popular game platform, and in fact, i dont even know any other
they even have a bonus dlc type thing that you could offer up?
i think usb or hard copy media is long gone, especially when you add world wide shipping to mix. some of us have internet speeds that would deliver the game faster then you could deliver a usb to neighbor. most of us have speeds that deliver faster then shipping.
time constraints are a big player too, many of us find our selfs with some spare time, and thats the time we seek to fill. id buy the game if i could download it and start playing in the hour of spare time, but pass it by and find something else (probably forgetting it) if i had to order it.
factoring in replay value is another big consideration.
long story short, id say err on the side of lower cost and have an extras bundle as dlc, maybe with more game content. a common thing today is getting you in the door with a low cost game, then offering expansions as extra.
once released, do you plan to add updates maybe a few new stories? or sell expansion packs?