I use both. If I'm doing a skinny scarf or a dishcloth or some other narrow or small project, I'll grab my straight needles so I don't tie up my circs. And sometimes if I'm casting on less than 100 stitches, I'll grab a straight needle for that, too.
But for big projects, I definitely prefer the circulars. And when I say a big project, I mean something with 100+ stitches.