NTD should not be a deciding factor in a choice between neons and tetras. It's quite rare...........not like
ICH or something. Take a look in the tank of where you're buying and look for signs of it. If you see any, don't buy.
Cardinals do have the better colors, but neons display more interesting behaviors and are more fun to watch, especially in the mornings when they are most active. You'd have to have kept both species to notice this.
I would not consider either difficult, not even medium. The issue with these fish is acclamation. The better of an acclamation process you deliver for them, the better. After basic water parameters are met, the acclamation process is everything to the survival of these fish. Done wrong, and they will drop like flies in the course of days or weeks. Here is how I do it, and my losses have been next to none..........
Firstly, I use an air line valve that has a knob to regulate airflow. I put this in between the air line to restrict the water flow to a mere trickle. Demand that the clerk put in a good amount of water. You're paying, you're in charge. Don't let them skimp.
When you get them home, empty everything into a container. I use any of my spare 2.5 gallon tanks (Sometimes I have to tilt them at an angle), but any clean container that will hold triple or more the amount of water they came in will do.
Place the container near the tank, bring in your line with the valve and begin the drip process. You want the water flow to be restricted to mere drops. Nothing more than a trickle of 2 drops per second type of thing. Doing it this way gradually brings the water and tank's temperature into the container. You'd want two or more parts of water to the one they came in.
While the drip is in progress, feed whatever fish are in the tank and after you place them in the tank, it should be lights out or you can cover the tank for less light.
Maybe some people do it differently, but this is how I've done it and it works. I do it for all my fish and shrimps. I have both cardinals and tetras.
I'd go with the neons, but both are beautiful fish.