The two restaurants you mention are both great, but they are very different.
1900 Park Fare is character dining in the elegant Grand Floridian hotel. The buffet has fairly typical American food. Boma is also a buffet, but there are no characters here. It is in the majestic Animal Kingdom Lodge. While there are some basic choices for kids on the menu, many of the dishes are of African heritage.
Your choice of where to dine could be influenced by many factors, including where you are staying, which park you will be visiting during the day, whether you have other character meals in your plan. However, all things being equal, I would choose Boma. Here's why ...
While the 1900 Park Fare food is good, it is similar to food you will find in many other places. The Boma food is both delicious and unique. I lived in Manhattan for a decade and ate there in the ethnic restaurants of many cultures, but had never encountered much African food until I dined at Boma. The flavors were wonderful and refreshing. And this is an experience that you are not likely to have elsewhere. Then, after you meal, you can walk around the Lodge and see all the African artwork and even go outside to see some of the Animal Kingdom animals.
Hope you have a great trip!