I do both for different comics.
I've done a bunch of oneshots over the years, and am working on one right now. They're a nice way to tell a story, experiment a bit with new characters and try out new settings and stuff. I've done scifi oneshots, romance oneshots, adventure oneshots, etc., etc.
As a creator, though, I tend towards longer storyarcs, though. My brain is just wired for picking an idea and expanding on it wildly. I actually find it difficult to contain myself and do shorter stories, because there's a part of my brain always excitedly jumping up and down and going "Look at all this stuff you could tell stories about!"
I always make sure to figure out my ending-point first. I don't necessarily need to end up exactly at that point, but it's nice to know what I'm aiming for.