When something is already planned or as Xavier said, something is already decided you use Going to. A: We are going to have a party tonight. Do you want to join us? B: Sure, I'd love to. I will bring some snacks. The party is already planned but person B has no plans to bring some snacks and he just decided to do it.

Thanks Alexandros!

In my experience, "going to" is much more common. Using "will" actually can be more conditional Alice: Do you want to go get something to eat? Mark: Yeah, let's go get some tacos. Should we invite anyone else? Yes: I will (I'll) call Sara! Also, "will" can put emphasis on a phrase Example: I will finish cleaning this house today if it's the last thing I do! I will not tolerate disobedient children in my house!

Thanks Xavier!

“Going to” is far more common in vernacular English, and it is sometimes shortened to “gonna” although this is informal. Usually “going to” it’s for something that is already decided whereas “will” is something decided while the person is speaking.