Fair question. The reason I built Decoy is that I wanted a good looking native macOS app. I hadn't really considered Mockoon to be honest, but looking at it now I can see a few differences that might be worthwhile in choosing between the two.
Decoy is 10 MB vs Mockoon's 329 MB and looks and feels differently because it is a native vs cross platform app.
Mockoon seems to have lots of great features such as online mocks and proxies for which you can pay their monthly fee or you could have a lighter weight app that does one thing well natively for a one time fee purchase.
I also notice that Mockoon calls home even when I don't have an account, probably for some sort of tracking. There is no tracking in Decoy.
So overal I think it is about lightness of the applications and UX.
Decoy is 10 MB vs Mockoon's 329 MB and looks and feels differently because it is a native vs cross platform app.
Mockoon seems to have lots of great features such as online mocks and proxies for which you can pay their monthly fee or you could have a lighter weight app that does one thing well natively for a one time fee purchase.
I also notice that Mockoon calls home even when I don't have an account, probably for some sort of tracking. There is no tracking in Decoy.
So overal I think it is about lightness of the applications and UX.