Hi8 = A higher quality version of the 8mm videotape format. Uses a higher luminance resolution to produce a sharper picture than regular 8mm videotape.
MiniDV = Mini DV is a video cassette designed for use in MiniDV digital camcorders. The picture quality of digital video (DV) recorded on a Mini DV cassette is basically identical or better to the quality of DV recorded on a Hi8 or 8mm cassette by a Digital8 camcorder. Mini DV can have up to 530 lines of video resolution for some camcorder models.
A digital video camera is better for videos, but the camera's in them are usually crappy, like 1 megapixel at best.
Digital still camera's will take very good pictures (up to 12MP), but can only take VGA quality (640*480) video.
You can't do both right now; it's one or the other.