If YOU pay the enroute charges then VFR on Top is the way to go.
Why? Isn't VFR on TOP an IFR using a visual (VMC) procedure?
If you have been sold that using VFR on TOP cost you less in charges, you have been sold a pup. As an ATC, I do nothing different to your flight plan if you request VFR on TOP, so how will the avcharges section even know you did something other than full IFR?
Bottle of Rum