A lot of it depends on personal opinion as to the colors, but there are some differences between the two species. Cardinals prefer warmer waters and can be kept with discus. They also tend to get a little larger. Neons prefer cooler waters (below 78F) and tend to stay smaller. Both do better in acidic water compared to basic water.
As for the hardiness, it depends on where your LFS get their fish from. A lot of cardinals are still wild-caught; this tends to make them more sensitive to pH and water hardness. But neons are commercially bred which can make them hardier; however there has been a lot of inbreeding with neons in recent years and this has weakened their stock. Some cardinal tetra are captive-bred, so if you can find some of those, they might be your best bet for hardiness.
If you're looking for a tightly schooling fish, look at rummynose tetra (Hemigrammus bleheri). They are some of the tightest schooling tetra out there. And they are rather pretty, too.
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