If you compare the two systems to each other when both are in proper perfectly working condition there isn't a big difference.
BUT finding a single piston setup that doesn't have a severely worn inner pad raceway groove is just about impossible.
The upper "Notch" in the steering knuckle where the "tab" on the inner pad goes
wears on the single piston system and the only way to fix it is to replace the entire knuckle.
On the two-piston setup the caliper bracket is a replaceable (bolted on) item
Generally speaking the early symptoms of problems with the single piston brakes is simply that the brakes don't behave the same way each and every time you put your foot on the pedal.
Inconsistant pedal height, brakes that "hang" (if your vehicle doesn't roll when you release the pedal at a traffic light) brakes that pull to one side or the other, but not the same way every time...
All can be traced to wear on that notch in the steering knuckle...
I did my two piston conversion 3 years ago and don't regret it a bit.
I won't build or drive another RBV without them.
AD