The fish are so close in mature size I wouldn't worry about that in deciding which species. A 24-inch tank would be minimum, and you could have a decent sized shoal (9-12) which is important for these fish. Neons do seem less resilient these days, assumed due to the generations of commercial in-breeding, while cardinals which may be wild caught (some tank-raised fish are available in Europe/UK now) tend to be hardier. Both require very similar conditions, and they are sensitive fish, to water parameters, water quality, and nitrates.
Soft water, and slightly acidic will promote better health especially in cardinals but neons too, regardless of the commercial raising. Temperature varies; neons prefer cooler water than cardinals. I would keep neons no warmer than 75-76F/24C, while cardinals can manage from 77-78F/25C up to 82F/28C. This is something to consider when selecting tankmates; cardinals are often recommended for warmer water fish like discus, blue rams, etc, and these temperatures would weaken neons fairly quickly.
Dim tank lighting is important; these are forest fish and they do not occur in watercourses exposed to bright sunlight. Floating plants can usually deal with this. [I am remembering your earlier thread on plants and the light, that is fine, but I would definitely add some good floating plants and you will see the colours intensify on both these species.] Thinking of the photos in that earlier thread, I would say that is well on the right track, but you could add some more wood, as chunks representing logs, tree trunks, roots, and/or branches. You could have some dried leaves like oak, maple or beech on the substrate for a very natural setting.
Byron.
