Is there a big difference in real life?

Often both are correct. The difference is clear when you decide at that moment (will) or you already had the plan (going to).

Which do I use for a prediction?

Going to if you see the evidence now. Will if it is your opinion, often after I think or probably.

Grammar, compared

Going to is for plans and what you can see coming. Will is for decisions now, opinions and promises.

Going to / Will
Going to	Will
I'm going to study medicine. (A plan I already have.)	I'll have the pasta. (Decided now.)
Look at those clouds. It's going to rain. (Evidence.)	I think it will rain tomorrow. (My opinion.)
I'm going to help her on Friday. (Arranged.)	I'll help you with that. (An offer.)

Going to and will compared
Point	Going to	Will
Plans	Decided before you speak	Decided while you speak
Predictions	There is evidence now	An opinion or a guess: I think, probably
Offers and promises	Rarely	I'll call you. I'll help.
Negative	I'm not going to go	I won't go. The car won't start.
After if, when	Never	Never: use the present simple

Spanish often uses the present for arranged plans: Mañana viajo. In English say I'm travelling tomorrow, or I'm going to travel.
Do not use will after if, when or as soon as: If it rains, not If it will rain.
Will is not the only future. Learners overuse it for plans. If you already decided, use going to.
Voy a is going to. Iré is will, but you hear it far less in speech.

Is there a big difference in real life?: Often both are correct. The difference is clear when you decide at that moment (will) or you already had the plan (going to).
Which do I use for a prediction?: Going to if you see the evidence now. Will if it is your opinion, often after I think or probably.

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