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When Baldur's Gate 3 came out, Steam's overall bandwidth consumption went from around 18 Tbps to 146 Tbps
(reddthat.com)
From Steam's self-published stats.
Baldur's Gate 3 could not be preloaded and weighed in at 125 gigabytes on disk, so when the game left Early Access at 11am US Eastern yesterday, Steam's bandwidth utilization shot up 8x over a span of 30 minutes. I know personally, I saw my download hit over 600 Mbps across a 1 Gbps fiber connection.
Kudos to the system engineers at Valve. It is mind-boggling that they have built infrastructure that robust.
I don't think the difference is worth considering. The computers running for hours actually playing the game would be the same and that's the bulk of the energy consumption. The spike from downloading it or physical distribution is probably irrelevant in the big picture.
The main argument in favor of downloading is, it's easier to provide the necessary energy in a cleaner way. You just need electricity, and you could power everything using solar or other "clean" sources. While the production and distribution of the physical copies will have to be done by boat, car, and potentially even airplane. And I think we are still far away from electric shipping boats.