“将来”、“未来”和formal, informal没有任何的关系。“将来”是“将要到来”的意思，而“未来”是“尚未到来”的意思，两个词是从不同的角度的在讲“尚未发生的事”或“还不是现实的时间”，因此，侧重点不同，与是不是正式与非正式没有关系。更细微的差别，不妨多看些例子。比如我们一般说“我的未来不是梦”，而很少说成“我的将来不是梦”，“我将来长大了，我会娶你的”，绝对不会说成“我未来长大了，我会娶你”。它们之间的差别，并不是不能解释，但要解释清楚也不容易，多多的体会吧。

“未来”多用于书面语，it is more fomal and used as a written word."将来"多用于口语，it is used in our daily life mostly , it is used in the start of a sentence and sometimes used as an adverb.

they are similar between "将来" and "未来" for most of the time. but "未来" is a formal word, “将来” is mostly used as the informal words. at last "未来“ is longer than "将来" , if they denote the future.

If you use the word as a adverb, the only choice is "将来"，because “未来” can't use as a adverb. If you use the word as a noun, "将来" or “未来” are both seclectable, their meanings are very similar, but "未来" usually refers to a longer time than "将来"。