Boma is all around the better meal.
The only exception to this would be if you were planning on going back to AK after dinner. Any time we have ever left AK with the intentions of going back, we always end up deciding not to. It's not so much the traveling back and forth, especially to AKL which is a 5 minute ride. It has more to do with the walk from discovery island to the bus and back. It's a long walk, especially on a full stomach. So if you plan on enjoying the park after dinner, I would suggest TH. If you don't plan on hanging around AK after dinner I would go with Boma.