幸亏，幸而是近义词，意思没有太大的区别。幸亏 means you are lucky, because you did, so, bad things do not happen. Both spoken and written language can use. 幸而 is not stronger than 幸亏, Usually we used in written language.

幸亏 means to thanks to something, or luckily. 辛亏有了你的帮助： thanks to your help 幸而 is the same as 辛亏， but not that common in spoken mandarin. I only use it when I'm writing something formal, but I can't remember when was it.

幸亏与幸而哪儿不一样？ 首先，幸亏与幸而是一对近义词。 【幸亏】：副词。表示因某种有利条件而避免不良后果：幸亏及时抢救，否则早没命了。 【幸而】：副词。幸而：幸而及时发现，否则要闯大祸了。 所以，在我看来，这两个词的区别并不大。