is it better to train light or heavy for a contest. and why?

You may have to be a little more specific since "heavy" and "light" are rather subjective terms open to a wide range of interpretation.

Generally speaking, I'd keep "heavy" weights in during your prep. as long as possible to help you maintain much muscle mass.

Hey Schmoomve!
Heavy in a nutshell the lighter you train the less muscle fibers that will be recruited (especially type IIb) The body will respond with a use it or lose it mentality.

The Shark! What's up?

Stl - Now that the shark has echoed my advice, you know it's legit