Well, Piranas must be the best clan for competitive gameplay, as Nightmare are mostly outdated. Also, it would help if you told us what the other half is. If you don't we can't give you advice about what Piranas cards that are good for you.
I prefer Piranas (maybe because i don't have Nightmare deck? lol) because they have better abilities, IMO. almost all the cards can be effective in bluffing, such as the +/- of the pills, +3 life of Rhed and some +/- of powers.
and i think Piranas have more solid Power cards than Nightmare