I think that polls get more people because frankly they take more time and effort to make. Sometimes I feel like one person makes a MILLION discussions in a day (usually they're just new) but if they take the time to make a well-thought out poll I like it better. It's also a bit more fun for me to get a comprehensive view of what everyone else has to say without reading the entire thread.
I usually only make a discussion post if I am looking for questions that need answers quick. So far I have only done a couple of polls, but I do notice they get more answers then discussions, unless it is on something good.
I actually tend to shy away from polls. About 90% of the time it's usually a five word sentence in the exposition space, and the choices don't list anything I agree with. Although it's an interesting way of finding out what's in the poster's head/attitude/values by what options they give.