Depends on the target platform. Both Unity and Unreal are very capable engines. Typically, Unity is slightly more geared towards mobile development, and 'easier concepts' whereas Unreal comes bundled with more robust out-of-the-box rendering features (to make, say, an FPS). For a 2d platformer, I'd go Unity, but for a 2.5d, either would work. Depends on your familiarity with said engines (few people have the luxury of working with both, as it takes considerably time to develop an expertise with an engine, and typically, the learning curve isn't paid work!).