Both are extremely overrated so they're both obviously successful animes.
Most people are disappointed by the second arc of SAO, but I was disappointed when I realised it was more about the forced romance than the actual game/iron fortress. The idea of SAO was what made me watch it, yet I wasn't expecting a love story fuck it of course I was. The second arc was disappointing by all means, but I think it could have been a great idea (apart from incest). I haven't even bothered watching SAO II.
Attack on Titan was a different style of anime. Not a typical highschool romance. It was different, but not great. The overhype of this anime is cancerous, especially since it's the only thing my friends at school talk about, praising anyone who likes it and shitting on everyone that shares the slight bit of criticism. I can't really say much about the anime since it was a while since I watched it, but it was good I guess.
It's hard to compare the two, and the only reason they are compared is due to the popularity. Both has their flaws, but I'd probably go with AoT.