Just a summary contrasting both expressions:
1. Decision just made
Oh, there is a fire: I’ll call the police immediately!
2. Predictions about the future (Science, weather forecast, fortune teller…)
We’ll have sun and fine weather tomorrow.
3. Opinion (I think…/In my opinion,…/I’m sure…/I’m afraid…)
I’m sure you’ll pass; don’t worry!
I’ll help you with your homework.