Neons and cardinals have very different sources that you need to remember to keep them alive. First of all remember they both sometimes come in batches that are doomed - I bought my cardinals in 2 groups - all except 1 of the first dies, all except one of the second lived, in exactly the same conditions. Also , try to buy ones that aren't too small - some I see can't be more than 6 weeks old, and these always struggle.
Cardinals - still largely wild caught - pluses - they're pretty tough, and have seen/survived lots of dieseases, no chance of inbreeding. Definitely prefer higher temperatures, soft acidic, water, but can be acclimated to 'normal' conditions. High kH and pH are associated with long term health problems and no chance of breeding. Caught from drying up pools in the forest, then stored in vats till they're sent out as an order, this means you could be getting fish that are over a year old.
Neons - farmed. Better adapted to 'normal' kH, pH BUT typically they're heavily dosed with antibiotics. Problems arise when they hit your tank with a deficient immune system. Also some chance of inbreeding. Do better at temps in the mid 70's. Typically sold at a very young age.
For my money I go cardinals everytime, and the larger , the better.