Well...the purple fire fish is going to be more expensive then the firefish. I have a trio of fire fish in my frag tank they are timid fish and will need to be in a tank that doesn't have agressive tankmates. They are also prone to jumping so some eggcrate over the top will keep your fish in the tank.
I really like the purple but I got firefish instead. The purple at my LFS
were $39 each where the firefish were $14.99 and I got a trio for $21 so....If you want to spend the difference I say go with the Purple.