That's a tough question - they are so different. Depends on what you are looking for, and where you plan to be that day. We really love both - Garden Grill has always been one of our favorites (our arrival night tradition!) and we tried 50's on our last trip and really enjoyed it. We have three boys, and they range from very picky (mac and cheese/chicken nuggets) to only wanting to order from the adult menu. They all like both, too.
We just returned a few weeks ago, and we think the food at Garden Grill has improved significantly. The food was great, character interaction was outstanding as always. They changed the menu a bit, and I know some people don't like the new offerings for kids, but the mac and cheese is still there and our kids liked the chicken drumsticks. Other posters have said that you can still get the old fries/chicken nuggets if you ask for them. Even our pickiest eater liked the food and did not request substitutions.
The food at 50's was great, too - especially the wedge salad and the pot pie. We thought the servers were very fun - and ours was entirely into the act. I used to avoid 50's because I thought the act would be annoying, and I even considered telling the CM to go easy on the act with our family. But I'm glad I didn't. Once we were there, it was really fun, and our server would break character once in a while with the adults and he was really funny then, too.
If I had to choose one, it would be Garden Grill because the food is very good and our boys love the characters. But it's hard not to recommend 50's! Again, just depends on what you are looking for and where you plan to be that day.