I haven't run a contest for a while, and I didn't do one with a prize in return for writing. In general you'll get a lot of traffic if you post to some of the more popular contest aggregation sites. A lot of these people will be professional or keen amateur compers, and in general the easier your contest is to take part in, the more entries you'll get.
My overall feeling was that these visitors weren't very valuable, because they weren't necessarily interested in the topic of my site, or in coming back for more unless it was for a prize.
However since your aim is content rather than repeat visitors, I'd say give it a go but start small. Offer a small prize for a small chunk of content. Personally I wouldn't touch a writing gig in the form of a contest, so don't expect much in the way of professional contributions. But maybe that's not what you're after?