They’re both fine

I don’t think that either of these would usually be perceived as conveying certainty. They would be recognized as encouragements, not as predictions. You can emphasize certainty by adding other phrases. I’m sure that it will rain tomorrow. There’s no doubt that the 2020 numbers are going to be lower than 2019’s.

If you’re in two minds about this then I have a tip for you: Psychologically speaking it depends on the person you’re talking to. If the person likes compliments and trusts in your words then it’s better to use going to, but if he doesn’t like exaggerating then it’s more suitable to use will because using going to in this case may have reverse effect on him. Hope it helps.

"You are going to be great" sounds stronger and more convincing.