HI Zodiac, your aquarium info states that you have a 50 gallon tank planted with no fish yet. Was this a typo or do you also have a 90g that you are also working on?
As far as your choices go, it may depend on what other fish you plan on keeping in that tank with either the cardinal's or Rummynose tetras.
I think you might find yourself surprised by how small a school of 10 looks in a 90g. I have 14 brilliant rasboras (which get larger than rummynose or cardinals) currently in my 90g, and I think the group is a little too small for the tank size.
That said, I'm planning to redo my 90 in the near future, and I'm leaning towards cardinals. The rummynose will school more tightly and more often, but I like the color contrast of cardinals against plants.
Depending on what other fish you plan on keeping, I say go for both......I would up the schools to 12 to 15, though.......I have been keeping african cichlids for about 20 years, but within the last year, I have been combining the african tanks to make room for community tanks with all little fish in..........I guess I got bored with the cichlids, but the exchange of setting up community tanks with plants (that don't get eaten or tore apart) has really thrown me back into the hobby headfirst