We've done both lunch and dinner and there are differences between the two meals. The entrees are different, but the soups are the same (there are some additional appetizers at dinner) and the desserts are pretty much identical. At lunch the carts that are used to bring your food to the table are the same ones that are used at dinner for the dessert presentation so you'll get to see the carts. Also, unlike most CS, at BOG even at lunch your food will be served on china and you'll have regular stainless (no paper plates or plastic utensils). Beverages, however, are in self-serve paper cups.
The other major differences are at dinner you have a photo op with the Beast after you finish dining and at dinner you can order beer or wine.
If you're more interested in seeing the interior, you can walk through all the rooms at both meals. One advantage at lunch is you can usually choose your own table and room.
We liked lunch better. It was a more relaxing meal because we could eat at our own pace while at dinner it was so busy our server rushed us through the meal. Although the ballroom is beautifully designed and decorated, its sheer size made it feel more appropriate to a CS venue which it is at lunchtime.
If seeing the Beast or ordering alcohol is important than go for dinner. If you're mainly curious about the interior of the restaurant and the lunch menu appeals, then have lunch.