Series or Stand-Alone

I have two young adult urban fantasy series currently being published by Jupiter Gardens Press. Both of them, Reality Shift and The Dark Lines, became series because several years ago, I had too much time on my hands and was going through some stressful situations, so writing became my escape. Reality Shift was supposed to be a series all along; The Dark Lines was originally one book that grew out of control.

I think it's pretty common for urban fantasy to lend itself to series. The author has often created an entirely new world for their story, and that world might have an infinite number of stories. (The Dark Lines might have been infinite if I hadn't balked at writing more than 30 books...Yes, there are thirty.)

Recently I received edits on one of my contracted YA contemporary novels. In the notes, the editor said he would like to see stories about two other characters in that book. In the original version of the book, those two characters' stories were major subplots, but the first editor I worked with at the publishing company asked me to cut out the subplots because she felt they detracted from the primary plot. I'd been thinking all along about writing those stories as separate novels, but I was balking because that would mean more series.

Apparently series find me...

What do you think? If you're an author, do you prefer writing series or stand-alone stories? And all you readers, what do you prefer?

I'm in a give-y kind of mood this week, so I'll draw a commenter's name on Monday to receive a copy of their choice of book from Reality Shift or The Dark Lines.