I came across this sentence:
四月だというのに、結構寒いね。 "It is April, but it is rather cold isn't it?"
Recalling what I learned about the のに conjunction, I can also make the sentence:
(Question) What is the difference?
In this case, there is very little difference, except in flavor. You shouldn't look at it as という vs. な because they can't be compared. The な here simply allows you to use のに with the noun 四月.
という, in my mind, is like putting air quotes on something. という gives the former example more flavor, I think. Source --> Opinions, one Japanese and one American. I asked the Japanese lady next to me and she agreed.
四月だというのに、結構寒いね。>> Even though it's April, it's quite cold, huh? Flavor: This super coldness doesn't feel like "April," you know?
四月なのに、結構寒いね。>> Even though it's April, it's quite cold huh? Flavor: I'm stating a fact about the weather this April.
というのに is used a lot, so keep an ear out for it.
A better translation of 四月だというのに、結構寒いね would be: ‘Even though they [we] say (that) it is April, it is rather cold’. The second sentence leaves out the bit of ‘they (or we) say’ and states it as a fact: 四月なのに、結構寒いね ‘Even though it is April, it is rather cold’. Since the nuance of ‘they say it is April’ exists in English as well there is no need to leave out という in the translation or confusion about how the two sentences differ.