There is a slight difference in meaning between "futur proche" (je vais dire) and "futur simple" (je dirai), but the main difference is that "futur proche" is used a lot more when speaking. It tends to be more concrete, whereas the "futur simple" tends to be more about plans or predictions. And yes, like David said, it's much easier to conjugate! :)

Roberta! Now that you know that je vais dire means je dirai, think about it as your friend. Instead of wasting too much brain power as a beginner, you can just say, "Je vais....", and then add the infinitive. OK, it's a little less formal (perhaps!!), but it saves you having to think of the future form of dire, voir and other verbs. Take voir for example. You have to learn that the future is verrai because it's irregular. But if you say, "Je vais voir" you can concentrate on the rest of your sentence without thinking a lot. Non? Just like, instead of saying, "Puis-je faire ça?", you can say, "Je peux faire ça?", avoiding the irregular word "puis". In fact I think most people say, "Je peux faire ça?" because even for French speaking people it's (as we say) a no brainer!

In this context, saying “Je vais dire” sounds more polite. In English we could say: “I would say that...” “Je dirai” is the future tense, so depending on the context, the meaning is different. If the person wants to say something now, but in a polite way, the person may use the conditional: “je dirais”/“I would say”. If the person wants to say something in the future, the person should use the future tense “je dirai”/“I will say”. Je dirais/I would say: Tu préfères lire ou écrire? Je dirais que/Je vais dire que je préfère lire. Do you prefer reading or writing? I would say that I prefer reading. Je dirai/I will say: Quand diras-tu cette chose? Je dirai cette chose demain. When will you say that thing? I will say this thing tomorrow. I hope that helps.

This is a little bit difficult to explain. They mean the same thing, but je dirai sounds more direct. In Italian- "je vais dire" = "sto per dire" "je dirai" = "dirò"