color wise i think they are about equal, the cardinals are generally more impressive because of their larger size. however if given the proper conditions, the neons will grow to the same size as the cardinals you see for sale. and yes you are correct, neons are not as demanding as the cardinals. my vote is for the neons.
To the logician all things should be seen exactly as they are. Any attempt at modesty is as much of an untruth as an exaggeration of ones ability.
The color on the cardinals is overall deeper, and the red stripe extends the full length of the body (only halfway on the neon tetra). I would limit it to 12-15 cardinals in a tank this size, as they are substantially larger as adults.