One of the main differences is how sensor size impacts a lens’s angle of view. A full-frame sensor delivers the same angle of view as it would on a 35mm film camera. An APS-C sensor “crops” the image by 1.5x or 1.6x (Canon). Attaching a 100mm lens, for instance, will look more like a 150mm lens on an APS-C camera. Micro Four Thirds sensors double the effective focal length, so a 100mm lens delivers a 200mm equivalent focal length. That can be both a positive and negative depending on your needs. If you want longer reach, the smaller APS-C sensors can offer it. If you want wider lenses, it’s easier to achieve on a full-frame body.