Assuming the audio tracks are the same bitrate, say 160Kbps, and only the video portions are encoded at different bitrates, the 8.6 Mbps video will have more color banding and artifacting than the 38.9 Mbps video. That being said, depending on the resolution of each video and the framerate, 8.6Mbps may not be that bad looking. It may not look as crisp as the nearly 40Mbps video, but it wont be unwatchable.