Personally, I prefer PDFs. Text is cool, but I like the speed and convenience of going from 0 to reading without the requirement of downloading anything.

if theres illustrations definately PDF

If the formatting is relevant, PDF.

If it's just text, nothing special going on, .txt files are good.

Usually I'd say text because it's easier to read no matter what type of computer/device someone is using. Though for your work I would say it's best kept in PDF format because the format used looks far more professional. Plus if any of the stories do have images PDF is definitely the way to go.
Though one other option is to upload the story as a PDF file, then copy/paste the story text into the "comment" box beneath the title while you're uploading, that way it will be visible on the screen to anyone no matter what format it has been uploaded in.

what Drag said.

if illustrations PDF is the way to go. But if it's just text then Txt files are aokay

I prefer PDFs or .rtf: both contain good formatting and are much easier to read than pure .txt.

Having access to both formats can be nice but PDFs are the way to go for sure.

Unless there's illustrations, txt is what I'd prefer. I read stories on my phone and stuff, and its usually a bit more tedious to read pdf's because the text doesn't all fit on the screen at a size I can read.