Try both and use whichever you like best. You can trust both of them to use good U.S. English.
VOA (Voice of America) is INTENDED for a non-U.S. audience, so it might be better for English language learners.
Their website offers a number of valuable resources for English learners.
VOA is an official broadcast from the United States government. Their charter says "VOA news will be accurate, objective, and comprehensive," but it also says it will "present the policies of the United States clearly and effectively." It's not broadcast in the U.S. and people in the U.S. do not listen to it.
CNN (Cable News Network) is a private news organization. It is VERY popular in the United States, and well respected as a news source. When you listen to CNN you are listening to the same real U.S. news broadcasts that millions of Americans listen to.