Efficiency of different solar panels
You'll need to compare like with like. Measure the panel sizes to obtain the area of the panel. Place them out in the sun side by side so they are exposed to the same amount of light then measure their power outputs.

Plot a voltage against current graph with different loads. Start by taking a no load voltage reading (V(open)). The maximum power output of the panel will be when the output voltage is half the open circuit voltage (maximum power transfer theorem). You may have to extrapolate this point on your graph. Find this current (I) and you can calculate the maximum power available from the panel for the amount of light falling onto its surface. (= I x V(open)/2)
Divide this power by the area of the panel to give you a Watts per square metre value.
Repeat with the other panel.
You now have two values that can be compared in terms of each panels conversion efficiency.
However even knowing this you are unlikely to be drawing maximum power from both panels. You could connect them both up and repeat the measurement to obtain a third watts per square metre value for the combination.