Both are known to be peaceful. Both are just two completely different methods. Which of the two methods suits you best depends on which one you prefer technically, whether you can get all the equipment and whether you have the right location.
Another point is that there are 4 different options for Nitrogen: exit bag, EEBD hood, SCUBA and SCBA.
The prices for all 4 options are very different and the possibilities to buy the right equipment are different in each country.
Nitrogen works faster. Usually it takes only a few breaths to become unconscious and only a few minutes to die. With CO both can take a little longer.
But there are also reports that people were unconscious after only 1-2 breaths. This may depend on the CO concentration in the room.
For CTB you need 10.000 ppm. A CO meter is recommended.