Organic Maps has a better and easier-to-use interface, especially for turn-by-turn navigation while driving.

OsmAnd is much more fully featured, especially if you are using it to contribute data to OpenStreetMap. With OsmAnd Live, you can download hourly updates to OpenStreetMap data, while Organic Maps updates at less frequent intervals. The app supports plugins for additional functionality, including trip recording, Mapillary street view, and various map views (such as nautical and ski views).

There's no harm in having both installed, since they have different strengths.