You are talking about two very different experiences. I would look at what both offer and decide on whatever criteria you are looking for.
Biergarden: Buffet restaurant with a live show. The food is pretty tame and I think everyone will have something they like. You sit at a table for about 10-14 and if you party is smaller you will be sat with other groups.
Garden Grill: Character meal in a slowly rotating restaurant. The menu is limited and I would recommend checking what they have all allears.net or wdwinfo.com where they have menus posted.
I have eaten at both and because they are so different can't recommend one over the other. I enjoyed both for different reasons.