Which one of those curses is worse?

The punishment is the same, a life sentence in Azkaban.

Is that fair?

A Dementor's kiss.

Is it a punishment at all?

I think that it all depends on how the curse is used. If it is used in self-defense against a bad person, then I think it does not deserve a life sentence or the dementors kiss. 10-15 years most definitely, if not more.

I believe that it is the same as killing someone with a gun. If someone is trying to stab you with a knife [like the Cruciatus curse, pain] then it is alright to shoot them in self-defense.

I believe that it all depends on the circumstanced. I 100% believe that the killing curse is the worst, althought there are circumstances [Neville Longbottom's parents, for example] where I change my opinion.