Future tense: Be going to vs. Will

We use “be going to” + bare infinitive to indicate

Future plan:            I’m going to have a checkup tomorrow afternoon.

Prediction:              It’s going to rain soon.

We use “will” + bare infinitive to indicate

Promise:                 I’ll call you again.

Offer:                     We will drive you home.

Prediction:              It will probably rain soon.

Prompt decision:    I’ll have my computer checked.