S. franciscanus is only found in the Sao Francisco Basin of Brazil, while S. brasiliensis is found quite widely across Uruguay, Brazil, Paraguay, parts of Argentina and Bolivia.
I don't know the origin, I could possibly find out but it doesn't make much of a difference for me. According to what I have read they are both very enjoyable fish, and very similar in all aspects.
Is there any significant difference in coloration as they grow up?
Aren't they just very similar in every single way?