Both shows aim to ground these stories in reality by making the leads just as skeptical as we are, but they don't skimp on the fantasy. "Once Upon a Time" does a better job of giving you an even amount of both worlds, and the design of the fairytale world is magnificently executed. "Grimm" is a little more clunky. The cinematography is still rich in colors and appropriate lighting, but it zeros in on singular figures, faces twitching in and out of transformation, like advanced stop motion.
The genres are also quite different. While it's clear that "Grimm" will be a supernatural police procedural, it's difficult to say whether Emma will put her bounty hunting skills to good use as some form of law enforcement in Storybrooke, or if she'll just spend most of her time wandering around town introducing us to contemporary versions of fairytale characters amd reluctantly bonding with her son in a family drama setup.
I'll watch a few more episodes of each, but my money's on "Once Upon a Time."