Not perhaps the wisest way to look at it.
The style of a story should carry the reader on like a boat floating down the river in the main current, without eddies or snags or still water. . . without the reader noticing the style. Ideally even when the reader is a writer, consciuos and aware of style and sentence structure.
Well, except for specialized purposes when it helps the story for the reader to notice. An epistolary novel, for instance. A clipping from a newspaper should be in journalese, a police report in its style, etc.
But most should focus attention on the tale. In the best, plain and fancy styles tend to merge -- with this proviso, that not all readers will be capable of reading a style smoothly. Some are harder than others, but there's a serious limit to how many stories you can tell in, say, a "See Spot. See Spot run. Run, Spot, run!" style. Vocabulary, if nothing else, will be a stumbling block, and sentence structures can also choke a reader, and both need stretching for some stories to work.
But at their height, plain and fancy blur together; only afterwards, when analyzing them, can you really tell them apart, with the metaphors and language and sentence structure of fancy being much more easy to note than the deceptive simplicity of the plain style at its height. Even so great a stylist as Lord Dunsany can tell so a tale smoothly that the words ripple by:
"Where are you going, child of men?" the troll asked.
"To the houses," the child replied.
"We don't want to go there," said the troll.
"N-no," said the child.
"Come to Elfland," the troll said.
The child thought for a while. Other children had gone, and the elves always sent a changeling in their place, so that nobody quite missed them and nobody really knew. She thought awhile of the wonder and wildness of Elfland, and then of her own house.
"N-no," said the child.
"Why not?" said the troll.
"Mother made a jam roll this morning," said the child. And she walked on gravely home. Had it not been for that chance jam roll she had gone to Elfland.
"Jam!" said the troll contemptuously and thought of the tarns of Elfland, the great lily-leaves lying flat upon their solemn waters, the huge blue lilies towering into the elf-light above the green deep tarns: for jam this child had forsaken them!
Of course, in their failures it's a lot easier to tell them apart, plain being flat-footed and wooden, fancy being purple and muddled. But they ascend toward each other.