Both fairytale shows have a unique take on the genre, and not all aspects are comparable, so I do not intend to choose a winner. I just want to point out a few differences, a few strengths and weaknesses that both shows possess that may affect viewership.
Longevity of Plot
One major advantage Grimm has over OUaT is the undefined enemy. We have yet to see the exact nature of the hexenbeists and the reapers. Keeping this mystery aspect is a powerful lure to retain viewers. OUaT has already set the evil queen as the highest “bad boss”, so once she is vanquished, there won’t be much else for the characters to do.
Per-episode Treatment
Also to Grimm’s advantage is the unpredictable plotline in individual episodes. In the beginning of each OUaT episode, once the audience has identified the correct fairytale, the rest of the episode is basically a waiting game until the tale is fulfilled (at least, that’s how I feel about the recent Cinderella episode). Grimm’s police case treatment is a good way of breaking up the stories, and I confess that I’m not familiar with all of the Grimm tales, so I feel like the storyline of Grimm is more refreshing.
Audience and Visual Appeal
Once Upon a Time is somewhat more glamorous than Grimm, if you consider the scenic presentation of both Storybrooke and the fairytale worlds. Not that there’s anything wrong with Oregon, but Grimm has a lot of dark, dank shots and night spooks. The mild gore is also inappropriate for children and therefore limits the show in terms of family appeal.
Also, I’m slightly disappointed that ABC subjected Lana Parilla into use as a sex symbol. It was expected, but I’m still picturing some Disney exec pimping her out as one of the visual draws of the show. They have a couple face cards in hand: James/PC, Emma Swan, the Sheriff and the Evil Queen. There’s no real sexual tension in the Grimm characters. Maybe a little with Adalind.
Jokes
Neither show is great with one-liners. At least Grimm admits to the cheese when Hank says “oh yeah, I went there” after making a buzzing joke during the bee episode. Did you know that the name Melissa (from the main suspect of aforementioned episode) means “beekeeper”? And Gilda for Goldilocks? I guess they try to be subtle, but some things have to be spelled out for fans to understand.
I have long complained about OUaT’s script. The last episode wasn’t so bad, but acting still seems a little forced. “Henry, what did I tell you… [dramatic, unnatural pause] about leaving shoes out on the staircase?!” Phew!
Characters
Alright, I know I said I wasn’t going to choose winners, but the real reason I enjoy any show is for the characters. Good vs. Bad, self-discovery bildungsroman, and real vs. supernatural happens all the time. The two characters keeping me in my seat are Monroe, the snarky reformed blutbad from Grimm and Snow White from Once Upon a Time. Monroe provides the much needed humor and story acceleration with his wit and skills; and essentially, the entirety of Once Upon a Time is epitomized by Snow White’s constant hope and love in the face of adversity.
KTC 
I only watch Grimm because it follows Chuck. So far I haven’t been terribly impressed, but then I’m not terribly impressed with most TV shows these days. It seems like all the good ones get cancelled early and I only find them years later on DVD. I completely agree that Grimm’s greatest strength is Monroe. Plot holes and not very great special effects weaken it considerably. I haven’t seen OUaT at all.
The plot holes and scripting are much more annoying in Once Upon a Time. I didn’t even want to bring up special effects, because you’re right, they’re atrocious in both shows and that’s about all that can be said. I consider them lunch-time sandwich shows, ones you watch on Hulu after they’ve already aired. I might be hurting (or rather, not helping) ratings by doing this, but until there is an OMG I MUST SEE THIS moment, my time has been better spent writing and reading. I feel this way about TV in general.