I'd vote for Garden Grill! We've been to both many times and they have both been on our kids' list of favorites for years.
Now that Liberty Tree discontinued characters, and two of our three boys are "Disney adults," we only do Liberty Tree for lunch. It's a convenient location and the food is pretty good. If it weren't for the location and our boys' nostalgia, I'm not sure Liberty Tree would stay on the list.
Garden Grill has always been at the very top of their list, and their favorite non-breakfast character meal. It's an arrival night tradition for us, and a nice compromise between characters and relatively good food. I think it's easily the best food available at a character dinner - always fresh, with a nice variety.
I think the menu is a little more interesting at Garden Grill than Liberty Tree - the bread is outstanding, the salad is very fresh and the sweet potato fries are wonderful at Garden Grill. I like the variety of meats and fish at Garden Grill and the veggies are good. The potatoes are great at both. The dessert is usually better at Liberty Tree, but we don't eat much dessert right after dinner.
Good luck with your decision!