Rain Dish only activates in rain (duh). In-game wise, there's nowhere that has rain permanently, but it's more of a cycle.
Weather - Rain
You could induce rain yourself, but that wastes a turn, and you might as well just give it leftovers.
Keen Eye prevents loss of accuracy, and in-game, there will be much more accuracy reducing moves used than competitively (where you rarely see any).
Competitively, Keen Eye is near useless, because you can just switch out and your accuracy will be reset. Usually, the only use for Pelipper in competitive play is to set up rain dance before it gets KO'd by an electric move. Considering that, Rain Dish is more useful.
In-game: Keen Eye
Competitive: Rain Dish
