We ate at both this past Sept & LTT was much better. IMO the food was much better at LTT - enjoyed the turkey, beef , mashed potatoes, mac & cheese, & rolls w/honey butter. I didn't care for the food at Garden Grill at all. I really didn't like anything on the dinner menu, except for the catfish & the flank steak. Overall, the food at Garden Grill was rather bland & unappealing IMO.
To offer the other opinion, we ate at GG this past April, and everyone loved it... both children and adults. Both food and experience were enjoyed. Not everyone enjoyed every type of food offered, but everyone found something (usually multiple) they liked a lot.
Having been to GG and LTT several times, we like GG a lot better. The food at GG seems lighter and fresher. Some of the vegetables come right out of the Land greenhouse. We've also had better character interaction at Garden Grill. At LTT, we've only had the characters come to each table once. At Garden Grill, they stayed longer and came around twice. I also like the rotating atmosphere at GG and there are more booths than tables so it is easier to take pictures.
My DH thinks that Liberty Tree is one of the best dinners. We are about to make ADRs there for our 3rd time.
We are also going to go to Garden Grill for the first time.