Oooo...tough one. We had two 7 y.o's with us when we went. And we went to BOTH!!! I just asked one and he liked BOTH and if he had to pick, he wouldn't be able to do so!
Biergarten was fun for them because there were a bunch of kids in our party and they went down to the dance floor and participated. Teppan Edo is entertaining right at the table. Will there be other children? If not, I'd go with Teppan.