They will both increase the fuelling by giving longer injector duration (i.e longer squirts). Any differences in the devices will be down to how much longer the squirts are at given points in the rev range. I'm not sure that counts as authoritative but hopefully it helps:) .

from what i can tell from looking at the picture in the Tunit brochure the V-PD unit has 2 connectors which interupt the ECU signals to the injectors , the ECU signals enter into the Tunit box which then sends out the modified signal to the injectors with more fuel being injected ,the other plug on the picture goes to unit and i think there will be pos and neg leads to go on the battery . AFAIK if you get the box fitted at the Tunit workshops in Chorley they will do a rolling road run and recalibrate the box for best performance , also mentioned in the Tunit literature is that it is adjustable by a manual rotary dial , so probably much the same as the Dragon box but as someone said is it worth over