Good question! I don't think it matters that much. I'd say it's okay to use both, but it depends more on the situation.
It's actually a bit like in English, so let me show some examples.
"Jag kommer att förklara" is a bit like "I'm going to explain", whereas "jag ska förklara" is more like "I will explain" (only a slight difference).
The difference is that "ska" is a bit stronger than "kommer att". It's kind of a bit more determined way of saying the same thing, therefore it's probably better to use "kommer att" most of the time.
Perhaps you could say that "kommer att" is a little more predictive, rather than determined. For example; "det kommer att regna imorgon" (it's going to rain tomorrow) is more correct, because you can't be sure whether or not it's actually going to.
On the other hand, "jag ska åka utomlands" (I'm going abroad) is showing a strong intention of doing something. If you used "kommer att åka utomlands" instead, it would be more like a notion that might or might not happen (maybe you don't have control over it, like a business trip). There is also a third way of expressing this, and that's "jag tänker åka utomlands", which basically means that you are planning to go abroad (in contrary to "ska", which means you already settled).
I know I'm not the best tutor, but I hope this helped you. Feel free to ask me anytime if you have more questions :)
