As other posters have said, both are good and it depends on which park you intend on being in that night.
For the 50's cafe, its a noiser atmospher, more energetic, if the waitress is a good one and plays her role, she will holler at you to get your elbows off the table, stop using your phone, sit up straight, etc. Its all in fun of course. The tables are closer together, and TVs are going all over the room showing old TV shows from the 50s and 60s. The food is great, huge milkshakes, fried chicken, I feel like I have to be rolled out of there.
The Garden Grill is a much calmer, quieter place which slowly revolves above The Land boat ride. There are no TVs, the wait staff does not yell, but you do meet Chip and Dale and Mickey and Pluto, so its not boring. Our party is all adults, and we love meeting these characters. Food is served family style, and its good too, the last few times we went it was beef and fish, potatoes, a veggie, stuffing, and cranberry sauce. It might have changed by now. Great cobbler dessert.
We always try to hit both places, you can't go wrong with either choice.