Actually from my readings on cardinals..not expierence, I read cardinals really should not be kept in hard water with minimal Ca2- and mg2-. The hard water can cause damage in the kidneys due to calcium salts causing blockages.
On a plus side, As It states most cardinals are wild caught(can't count on this)
Neons, can tolerate hardness in water(from expierence), and once you get past the first few weeks, can live years and years in a tank.
But neons can also contain the deadly Neon tetra disease.They also , recently in my purchases of them are not as hardy as they used to be around here.
Just some thoughts.