Also, keep in mind that you have to chill whatever size you boil. If you have a way to chill 5 gallons of boiling wort, like an immersion chiller, then I'd go with the 5 gallon boil. If you have to chill in an ice bath, I'd go with the 3 gallon boil. It takes a long time to chill 5 gallons of boiling wort. For 3 gallons, you can stick it in an ice bath until it's under 90 degrees, then top up to 5 gallons with cool water.