So you have some questions about which unit is right for you. Getting the right fit for your tank is vital health of your tank.
Swing on over to the manufacturers website for an informative guide that will guide you on which cooler is the best one for you.
Remember It is better to pick a unit that is going to over perform instead of under perform. My car engine analogy goes like this.
A 2009 Chevrolet Corvette ZR1's LS9 V8 Specifications(specs) are pretty dang impressing if you ask me. Yes you could run this bad boy pretty hard and fast for quite a while. Now what is the max speed you can go ?
Top speed of the Corvette ZR1 2009 is approx 200 mph give or take. Yes the car can do it but would you run this beauty @ full 600 plus horsepower it can go all the time? The answer is no. The reason is 1 you will decrease the lifespan of the car considerably and 2 energy costs / gas costs.
The same goes with operating your jbj chillers in your aquarium.
If your jbj chillers are running in your marine tank running 24/7 @ full capacity because you chose one that is just that is almost at max capacity you will be using more electricity and decrease the lifespan of this unit as it will contantly be running trying to keep the water chilled.