I've seen a fair amount said about the two shows. Obviously, this is a case of one network having gotten wind of another network's show. It's not so much "copying" that goes on as that the studios have tons of projects that have been proposed at any given time, so when they hear "modern day fairy tales" then can quickly greenlight whatever they have in the proposal stage that's similar.
The similarities aside, the two shows are radically different, and I think they'll appeal to different crowds. Grimm looks to be a police procedural with fantasy elements (Buffy the CSI Werewolf Slayer) while Once Upon a Time is more like LOST in many ways (large cast, stranded in a strange place, flashbacks) and definitely the more daring concept.
My problem with both series is that the writing is good with a few painful bumps along the way. Fantasy is hard to get right and fantasy melded into the modern day is even harder. But both Jane Espenson (Buffy: The Vampire Slayer, Battlestar Galactica, Torchwood, Once Upon a Time) and David Greenwalt (Buffy: The Vampire Slayer, Angel, Grimm) are alumni of the genre, so I expect that they'll both bring what they have to bear over the long run.
I've heard people ask, "where is Once going to go once everyone remembers," but that doesn't concern me. A few things to keep in mind: while the major first-season villain is likely the Evil Queen (Lana Parrilla), I very much doubt that she will end up being the "big bad" for the series as a whole. Right now, my money is on that being Mr. Gold (Robert Carlyle) or perhaps whoever he's working for...
Grimm is also setting up quite an interesting set of villains. Without spoiling things too much (as there are some heavy spoilers to be had) the villain that is introduced at the end of the pilot will obviously put a spin on the series. I imagine that villain is going to be a one-to-two season focus and then they plan to broaden the scope of our hero's "hunting".
I'm probably going to stick with Once for now, but I'll come back and check out Grimm once the DVDs are out.