Comparing negatives (It is nice not to be rude.) vs. (It isn't nice to be rude.) [duplicate]
Solution 1:
Both sentences are formally correct in their meanings.
Compare:
It is nice not to be rude.= It is nice to be polite.
It isn't nice to be rude.= It is bad to be rude.
Though, from the stylistic point of view, the first sentence sounds strange because 'nice' is a 'positive' adjective and needs a 'positive' word (for example, polite).
Solution 2:
"I decided not to come" = I made up my mind that I would not.
"I didn't decide to come" could be followed by something like "...until I heard that you would be coming too" or "...my friends just bundled me into the car."