Cardinal Tetra or Neon Tetra?
These two somewhat similar-looking fish have some important differences. Neons are generally considered hardier, but given the pH of the tap water many people have, better luck may be found with cardinals. There is also neon tetra disease plaguging some farm-raised neons, so you have to consider that as well. In either case, make sure you buy active, healthy-looking fish, preferably ones that have been there at least a week (as opposed to delivered yesterday). Acclimation is stressful to both types of tetras, so go as SLOW as possible and make this comfortable for them. You don't want to lose your tetras a week after you buy them'
Cardinals are generally also more expensive. Neons are much easier to breed. They stay more or less the same size. So while these fish are similar looking, they are some key differences between the two.