I honestly can't tell the difference between the two...
I would classify Science Fantasy as being a degree of science realism removed from Soft Science Fiction (which is in turn a degree removed from Hard Science Fiction).
Hard Sci-Fi seeks to remain as true as possible to known scientific understanding and principles, or uses such an understanding as the basis for speculation or extrapolation (hence "hard"). Soft Sci-Fi still operates within the realm of the scientifically feasible, though it takes much more liberties with actual scientific understanding (so it's often closer to pseudoscience). It is less concerned with "hard sciences", like physics or engineering or chemistry, and may focus more on sociology, psychology, and so forth.
Science Fantasy, as opposed to the former two, melds elements of Sci-Fi and Fantasy together, and is thus less bound by the limits of science realism than Soft Sci-Fi. It may liberally combine elements of science and magic.
An example of the distinction between subgenres would look like this:
Hard Sci-Fi > Soft Sci-Fi > Science Fantasy = Primer*** > Star Trek > Final Fantasy
With all of the above in consideration, Metroid strikes me as Soft Sci-Fi, since it uses the concept of scientific laws, principles, and so forth as a basis for the laws of its fictional universe; however, those laws, while inspired by science, may not be scientifically viable in actuality (e.g. Chozo technology, Power Suits, Morph Balls, etc.). However, it doesn't really implement elements of the Fantasy genre into its world or stories.
***Primer is an indie film known for its hard sci-fi approach to time-travel. Since the story stays close to the realm of science realism, it can qualify as an example of a hard sci-fi movie (and the film was written by a mathematician, which further reinforces the hard science approach).