Personally, I wouldn't use either to whiten my teeth. Lemon juice is much too acidic to apply frequently to teeth. As for baking soda, it might help to remove some stain.
Best way to whiten teeth is to first be sure your mouth is healthy. Then see your dentist to get a good cleaning - some of the dingies may just be cleaned away.
Then speak to your dentist about safe ways of whitening teeth, be it at home or in the office.
Remember, if you mess something up, you do not grow more teeth.