I tend to skim stories, so often a lot of the finer details are missed unless/until I actually go back and read every line of the story. Usually I find longer stories to be more tedious and would rather see a series of short stories set with the same characters/in the same universe than one long story.

I've also found myself to be a lot less patient when it comes to stories, especially written ones. If the writing has too many holes or inconsistencies I stop caring pretty quickly. A shorter story has less of a chance to have those problems, so I prefer short stories.

As well, because of some experiences I've had with reading books that never seem to end(I'm looking at you, Dresden Files!), I'm much happier reading a story I know has an end rather than reading a story where I'll eventually have to wait for more content. If it's a long story, but all of it is posted at once then I'm fine with it. But if it's a long story where I need to wait an unknown amount of time to get the rest of the story, I'd rather skip it, thanks.

I'd say for porn, generally short stories are better than long stories.