Join Date: Mar 2014
Location: Arizona, United States
In my personal experience;
I think that Neon Tetras are more appealing, the way that the red doesn't go from "head" to "toe", and also how they have white mixed in as well. Gives the tank a variety of colors, not just the blue/red combo that is seen from just cardinals. For the size difference, they aren't that noticeably different in size, maybe a few ~.5" difference from what I've seen first hand, and they both need to be in schools of at least 6.
For being hardy, I've personally only had Neon Tetras and they seem extremely hardy. Plus after awhile they get used to you, and also have a personality (small, but noticeable). I do make sure my water parameters are always in the right zone, but if you make sure that they are kept well and feed right (in my tank, there is a "alpha" neon tetra that loves to steal food from others, and bully/take food from the smallest one) they'll do perfect.