I have had reds that were very interesting and some that were pretty skittish. If you really want to get into piranhas, there are so many different types that deserve looking into. For pygos like Nattereri (red bellies), you can mix it up and get P. Cariba or P. Piraya, both are beautiful fish. Or the yellow variant of P. Nattereri is much thicker and a great fish. I have a mixed pygo tank with larger Cariba, Piraya and both yellow and red nattereri....a great tank. My favorite piranha tank is S. Geryi...a totally different type of piranha and a wonderful tank. For a single species tank I have a 16" S. Rhombeus that is just a beast and a wonderful show piece.
Moral of the story...there are a ton of different species of piranhas out there that can give many people what they are looking for in a fish.