Just got done watching all of Gravity Falls as a way to fill in the Steven Universe Hiatus. I think its great, and the way they build up the suspense and mystery of the books is awesome, and the show is definitely more consistent in quality throughout the whole series, but I still think Steven Universe is my favorite of --erm, whatever these new types of cartoons are called. (Not sure where to post this since its about both shows but whatever I'll post this here since I feel uncomfortable saying a different show is better in a thread about another show lol)
I feel Steven Universe has way more complex and emotionally deep character moments, which is what is really important to me. Don't get me wrong, Gravity Falls has some great moments, like Soos realizing his dad is a jerk and he doesn't need him and Wendy telling Dipper she's too old for him. but they just don't compare to stuff like amethyst saying 'I didn't ask to be made' after fighting pearl in the Kindergarten or Pearl saying 'What would Rose think of me now' and Steven replying 'Well.. I think you're pretty great!'. Also the humor in gravity falls occasionally gets on my nerves. Things like the security guard in the time traveling future falling for mabel telling him she's his great great great great great grandma. I dunno it just feels too forced a lot of times and heavily reminds me of The Fairly Odd Parents Which I dont like. There's still really funny moments though. The climactic moments of gravity falls are awesome, and Not What He Seem's ending had my heart skip a beat since it was something that was hinted at for so long, but it still wasn't as epic as Garnet singing while fighting Jasper in Stronger than You. Maybe that's just because I really like the music in steven universe though lol.
Either way both shows are great and in the end im a fan of both now