They do different things.
Small one evens out your voltage fluctuations in your ignition coils. Makes it smoother.
Big one increases the strength of the sparks. Theoretically, this is one that will give you gains. Factory ignition coil voltage is about 13.6v. Your ignition coils will burn out at about 24v. This thing can boost up to 60v (!!). So you adjust your voltage to the point where you have both gain in power and relibility. The range is 16-19v.
I dunno whether the one shot price is $700. I paid $730. But I guess handome people who can operate aeroplanes tend to get better prices...