I've been to Iceland 5 times (so far) and trying to compare August in Iceland to any time in Antarctica is really a reach.
It seems you have made up your mind here. I hope you are not disappointed in your cruise due to not being able to be on deck for scenic cruising opportunities, like the Lemaire Channel, and needing to be hunched over in the Zodiacs, missing incredible views of icebergs, penguins, seals and whales.
And with that, I believe I have given you my informed opinion in full and will not bother you again with my knowledge.