The SV3ORA one should give better performance if you can manage the construction accurately. The one on qsl.net is easier to build but looks to be a very old design and I doubt you would find exact parts these days. I would guess it's around 40 years old. For low frequencies the qsl.net one should be OK but for anything above say 30MHz use the SVORA design.
It should work down to quite low frequencies, the only thing that happens is it becomes a little less sensitive as the reactance of the 300pF capacitors becomes significant.
The variable capacitors are I would guess made from twisted insulated wire and probably are no bigger than 5 - 6pF. With a high frequency input ( >100MHz) and a good load resistance they can be adjusted (twisted more or less) to give lowest output. They are there to compensate for differences input and output capacitance due to construction differences and component tolerances.
At low frequencies, provided it's built properly, the match should be good anyway. It's only at higher frequencies it might need a 'tweak' to make it symetrical and once adjusted it should be good at all frequencies.
The change as the frequency gets higher is because of two factors.
1. small changes in symetry of the construction and minor differences in the components play a larger part n the measurements and
2. the feed and load impedances are more critical. For example, is your load resistor rated to the UHF frequency you are testing? Proper UHF terminating resistors look like plugs with no cable hole, the resistor is carbon composition and entirely inside the plug body.
I suspect the problem is those capacitors. The imbalance produces a greater error as the frequency increases. I would remove them (or at least one of them) and try a twisted wire capcaitor as a replacement. If you repeat your tests and adjust the capacitor by twisting/untwisting the wires you should be able to get a better balance and in theory it will still be OK at lower frequencies too.
To give you some idea of how critical it can be, a few years back I worked in the US for a major telecoms company on the development of cell phone base stations and in the antenna matching units we even had trouble with the uneven thickness of copper in the PCB tracks.
Looked at your web site - very nice! Where in Greece are you based?
Sorry - I've been away for a few days and trying to access the Forum using mobile phone with bad reception.
The monitor and output ports are voltage sources so I wouldn't try to load them with a low impedance. If you want to monitor them and invert the signals at the same time I suggest a dual op-amp with high input impedance such as the TL082. This will also allow you to amplify the voltages if it helps. If you connect the TL082 with a dual supply you can measure down to very low power levels at the monitor port.