We've got two stop-motion films in theaters right now, and while they're both in the same basic genre (PG-scary, appealing to kids who want a good "kid-level" scare) they're significantly different.
Summing up: Paranorman is a paranormal horror comedy, while Frankenweenie is an old-school monster movie wrapped around a running homage to old-school monster movies.
I liked Paranorman quite a bit better. The story had more heart, and it was spooky enough to chill me a little -- especially when our protagonist had his solution in hand, but my inner screen-writer knew we were only in Act II, so something was about to go horrifically wrong.
But Frankenweenie had me laughing more. Not raucous, comedic laughter, mind you. It was more along the lines of pointing at the screen and saying "hah!" with a smile. "I see what you did there. That was Gremlins, 1984." The film obviously takes much of its structure and theme from the original "Frankenstein," but nods cleverly at numerous other films from that era and others.
For the record, the science teacher in Frankenweenie is by far my favorite character from either film. I want my kids to study science from him.
Both films were beautiful. I like my movies in color, so Frankenweenie fell short for me, but I appreciate the craft that goes into taking a limited palette and exploring every last monster-infested corner of it. I saw both films in 3D, and they were both really solid in that regard. Paranorman did more with it in terms of effects, but Frankenweenie exploited it really well in establishing focus, pointing up emotion, and communicating scale.
(So now you know: I don't like black-and-white, and I don't mind good 3D. Any film majors, cinematography historians, or students of the critical school who wish to take issue with this can go suck a rotten tomato. I suggest adding salt.)
Both films are worth watching, and are probably great films to see with your kids. My 9-year-old son turned to me at one scary point during Paranorman and said "I wish we hadn't come to this. It's too scary." So I held his hand tight, he got past that moment, and he really liked the rest of the film, including the Act II disaster I mentioned above. He even twigged to the fact that said disaster was, in point of fact, impending.
"Dad, I don't think this is going to work. If it works, the movie will be too short."
Paranorman comes in at #9 for me this year. Frankenweenie is at #17. I'll cheerfully watch both of them again on DVD with my kids.