Okay, so what would be your first choice? I must admit I’m torn as I love both.
A lot would depend on time available and I love reading, as well as writing, the short forms of fiction. I love the idea of crystallising a whole world in a few hundred words or so.
Short story and flash fiction collections have the huge advantage of giving you a chance to taste an author's work and see if you like it before you read their longer works. From a writing viewpoint, it is lovely to be able to write and submit short stories and flash tales to different markets and competitions while working on longer term, bigger projects in the background.
But for the creation of a huge world it’s hard to beat the three volume epic and The Lord of the Rings is the definitive version of that for me. (Just don't drop the book on your foot!).
It is a little ironic that, as a flash fiction writer, I veer between the quick read and the very long one! But then maybe that is why. There are times I need to read the exact opposite of what I do.
Hmm… I guess that means I ought to get around to War and Peace then!