AVCHD and MP4 are variations within the h264 standard. With AVCHD you can record up to a bit rate of 28Mbps. With MP4, 35Mbps. Each one also encodes the audio with a different codec. If you want to shoot HD, and not 4K, I would suggest MP4 at 1080p, 50fps, 35Mbps. A lot depends on the camera and the quality of it’s optics.
