Go with the Exo's but not 20 I'd go with 10......I have a species tank with Exo's.....great fish they have been known to take down fish twice their size.
Hiya Kurt-inge, welcome to MFK.
You will need a bigger tank soon, crowded piranhas don't stay crowded for long, they start eating each other. Sort of like New Yorkers, heheh.
I say go sanchezi, my p would chase my finger around like my cons. Hey if you can get s. Geryi then you'll be the owner of a sick lookin p. Silver with a purple racing stripe from the tip of its nose to its tail. Very kewl.