I prefer all at once, I don't mind if they have some length. My own trip reports are very few and far between, but I always post them on AA, and infuse them with photos. While I do try to watch my length....usually I just watch them run long, because...to me, there are two elements I'm trying to cover: 1) Some storytelling within the narrative of a trip report, otherwise it is no more compelling than a weather report, 2) some facts and figures, by which I mean when I am planning a trip, I look up trip reports to see what I might encounter, and I try to return the favor when I'm putting one together.
So...that's what I do...but I like to see that in others' trip reports - storytelling plus some information about what I might expect on that particular route, and if that takes a few minutes to read through, no problem.