As one Archipelagan who has limited exposure to non-scientific readings, I vote for short stories. I doubt we will be able to pick a book all of us have read (for understanding) and I suspect I would be the limiting factor.

With short stories, we may all be able to read one during an Archipelago cycle and make comments by the next. For example: Griffin & Sabine by Nick Bantock or The Last Unicorn by Peter Beagle.