i would go with the 6'' only an extra $50 i think.
i added a 4'' piraya a month ago and sold 2 8'' reds only keeping my 2 5'' + reds as its not worth the risk on such an expensive fish.
you could grow it out in another tank i think thats what scarfish is doing at the moment with a 4'' piraya, i would have done this but dont have a tank big enough
definately get a piraya though they are amazing fish and rons fish are always exellent condition. the one i got was shipped for @ 30 hours before i got it home and was swimming about within 45 minutes of getting in the tank and eating after 2 hours
dixon