I live in Portugal, not Brazil, and here the rule is that the possesive NEVER replaces the article. Quite unlike English, if the sentence would require an article without the possesive then the article HAS to stay (o meu livro, o teu amigo, a sua amiga, etc). I believe that in Brazil it is more flexible.
As for Brazil: most of the time, the article is there to specify a certain noun. And it is a rule that all nouns must be specified (unless it's one of those tricky exceptions).
The thing is, the possessive is also a specifier. So it is sort of redundant to specify a noun twice, unless you really want/need to put emphasis on that.
The official rule is that it is optional. And then some people are used to using them, some aren't. I would say that most of the time, specially when speaking (when everything is simplified), we simply avoid it since it's not necessary C:
Grammar-wise there's no difference: both are proper formal ways to use the simple future.
Now, in conversation/daily use... Years ago, vou pagar was considered informal, colloquial speech. Then it was used so much that it was "promoted" to be just as formal as pagarei - and this is not recent, it's been like that for decades.
Because of that, still to this day, some people still argue that vou pagar is informal and you shouldn't use it in a situation that demands some respect or formality. But that's a school of thought, it's not true to the reality - at least not here where I live. I'm sure the region plays its part too, but I wouldn't worry too much about it - both forms are used/known everywhere and I would say vou pagar is more common C: