Short stories or novels? Both, please.
Novels give me the opportunity to sink into a whole new world (even if it's this one), to live life through someone else's eyes and to know everything about them. I get to let the character lose in a world, through obstacles at them, give them subplots and secondary characters, to watch them grow and learn and adapt. Apparently, I get to like them so much that I'm mostly inclined to grant them a happy ending.
Shorts give me the chance to walk a bit more twisted path. I get to focus on one big scene, one character, one big obstacle. I can tell the story in ways I'd not typically use for a novel, experiment, and try new genres or methods without a big time investment. The challenge is to pack a lot of bang into far fewer words, without the sinking into the world, without the getting comfortable with the character, without meandering through the plot idea to wait for it to really get rolling. Shorts really help me focus on the story itself, and that in turn gives me better focus when I'm working on novels.
Writing shorts helped ease me into the idea of writing shorter novels. I'd always been a long involved novel kinda girl until the short story bug bit me. They gave me the freedom to admit that I only had to write the story that needed to be written without having to complicate things with additional subplots and characters. I like to write complicated. I still do if the story needs it. But I don't need to every time. Sometimes a beautiful story is a simple one.
One of these days I'd like to venture into the novella zone. That's one length I've not tried yet. Who knows what I'll learn there?
What about you? Short or long?