Nitrogen would be a lot safer for everyone else around. You can release lots of N2 in a reasonably ventilated room without harm but that's not the case with CO.
Everything I’ve heard about hypoxia indicates that it’s a neutral or pleasant experience. You lose your higher mental functions first so you don’t panic or worry about what’s going on. You feel fine until you keel over. The fact that this isn’t the standard way to do it is a clear sign to me that the people making these decisions aren’t interested in making it quick and simple.
There are symptoms. It's only a nice, calm death if you're sleeping - so you'd want to do the usual suicide technique of sleeping pills + carbon monoxide.
