I have always been told that a good rule of thumb for ample filtration is ten times the tank sizer per hour. EX 30 gallon tank= 300 gph. It might be overboard but its better to have to much then to have to little unless the flow rate is pushing the fish around. I have 500 gph on a 75 gallon community tank and it is doing fine, but the bioload is a lot less then that of 6 pirahna's.
I would go with the bigger tank, multiple pirahna's are to much for a 30 gallon tank IMO.