Grimm is, in general, darker. There's fewer recurring characters, too. You've got to love the main character, Nick, and his crew if you're going to enjoy it. This makes sense because Grimm's fairy tales were much darker and often more fatal than the Disney version. This is more Sci-Fi and supernatural to me.
..Which brings us to the ABC version. Disney owns ABC so the portrayals of these characters is much closer to what you've probably learned as a child. The series definitely has more characters, which can get confusing. However, it centers more closely around Snow White, the evil queen and Emma. I feel like this is more fairy tale-like, but it's also grown up. There's also a lot more.. CGI in general. With Grimm, CGI and effects focus around were-characters. ABC uses more CGI for things that are happening.
I watch them both and get something different out of each. Hope that helps.
FYI, you might consider changing the title to be more descriptive. (=