Just two days ago I read a novelette by Algis Budrys, Wall of Crystal, Eye of Night. Mars with ancient cities and living Martians. Published in Galaxy, 1961, when almost nothing was known about the surface of Mars - Mariner 4 wouldn't make its flyby of the Red Planet until 1965. Still, there must have already been strong doubts, in 1961, that Mars was inhabited by any life form higher than fungus. So Budry's story had at least one foot firmly planted in Science Fantasy. This didn't keep it from being recognized as a great story - it was included in The Tenth Galaxy Reader (1967, two years after Mariner 4), Door to Anywhere (1970), Alpha 2 (1971), Galaxy: Thirty Years of Innovative Science Fiction (1980), The Arbor House Treasury of Modern Science Fiction (1980), Great Science Fiction of the 20th Century (1987), and The Great SF Stories #23 (1991). I'm sure if we read through the list of Hugo and Nebula winners and nominees, we'd find many more examples of Science Fantasy that were highly thought of - especially among the novellas, novelettes, and short stories.