Of course, I'm not the only one to pay attention to their very close release schedules. Teresa Jusino over at Tor.com posted her response to the pair, which I intentionally didn't read before writing this. (However, most everything over at Tor.com is worth reading, so I'll blindly recommend going and comparing her notes to mine.)
Here's my assessment: Grimm is actually an urban fantasy in the UF Noir style (ala the Dresden Files and others) that uses fairy tale elements for its paranormal component. As it's made by some of the writers who were on the teams of Angel and Buffy, the similarities don't entirely surprise me: in some ways, the series strikes me as Buffy if the core audience being targeted were mid-career adults rather than teens and twenties. It's also a cop show, and I suspect it may end up feeling like a cop show with paranormal elements rather than a fantasy with cop show elements. I think that may work in its favor.
Once Upon a Time, on the other hand, is a fairy tale writ long. In the tradition of fairy tale retellings like Bill Willingham's Fables comics, Sondheim's Broadway musical Into the Woods, and (most recently) jimhines's Princess Quartet, Once Upon a Time takes the familiar stories and twists them, just a bit, recasting real fairy tale characters as unknowing modern-day humans, for whom time has stopped. The only one to know about the Curse that has brought them out of their fairy tale reality and into the real world is Henry, a little boy, who is the biological son of the destined hero (the daughter of Snow White and Prince Charming), and the adoptive son of the Evil Queen. The hero herself, Emma Swan, is a tough girl loner who doesn't really believe in Henry's story, but finds herself drawn to the child. The cuts between the fairy tale backstory and the modern break-the-curse plot honor the romantic atmosphere of fairy tales -- and, thus far, aside from some off-stage cutting out of hearts, are doing it in a pretty tame way. Sure there's swordfighting and sorcerous battles, but it's not the sort of gritty and dark flavor that Fables and Into the Woods brought us. The fairy tale versions of the characters don't have anywhere near the depth they do in Jim Hines's books.
But that may be part of the point: while Grimm is, from the get go, down in the brutal side of those beloved and scary German folk tales, Once Upon a Time is Disneyfied, right through the use of the name Melificent for the wicked fairy who cursed Sleeping Beauty. Because the team of Once Upon a Time, was also part of the team on Lost, there is some worry that the fairy tale elements may end up being a lie after all -- but from some quick research on what the creators wanted to bring to the show, it doesn't sound like that's their intention. But while I think Grimm starts by knowing what it is, as a show, right from the very beginning (and, by virtue of the Monster/Villain-of-the-Week potential, could go on for seasons), Once Upon a Time launches its major plot in episode one, and that full plot arc needs to be resolved in the first season to feel like the story is going anywhere. The quest structure could work in its favor if they can raise the stakes for Season 2 -- or it could mean that the show has a one season maximum until we all get back to happily ever after.
It may sound like I'm being hard on Once Upon a Time here; I am being pretty critical, because it's a subgenre I'm invested in. But I'll definitely say that after watching two episodes (I've only seen the pilot of Grimm), I'm drawn in enough to keep watching, at least until the end of the season -- or however long it survives this season! I have a feeling that in the current TV climate, Grimm with its gritty appeal and its ambiguous morality will find its audience with no trouble at all -- and unless things get too scary for my fluffy-bunny-horror self, I'll be sticking with it.