The difference begs expounding.
A fashionable gentleman is aware of and able to own a trend. He will look fitting and impressive in his ensemble, embodying the absolute nature of say, a suit and tie. He will own his look. A fashionable gentleman is sartorially solid. He is comfortable in wearing clothing as it comes, as it is intended to be worn, few frills or strays from symmetry and beauty.
A stylish gentleman possesses a critical eye. In contrast to fashionable man, a stylish man will construct his outfits from a loose canon of clothing, usually acquired through more organic avenues. He may exist at the center of a trend or miss it altogether. Most importantly, his attention lies in personal detail. Through a brooch, a flipped cuff, or an ornately knotted scarf, style is achieved through detail. Personal detail. It is the underlying umph of an outfit that can exist left of center.
However varied these approaches may be, they are not mutually exclusive. When married, fashionable and stylish breed a wonder baby of personality and awareness. Whichever camp you deposit yourself in, keep expanding your sartorial horizons. Draw from personal expression while respecting the eye of the designer.