What is more important for a solar panel, voltage or amperage?

voltage is a potential difference

It's the current that does the work

ststic electricity is mostly voltage, but with negligible current.... when you put your hand near an old CRT TV and feel the hairs standing up, that is because of the voltage...... but it is of no use without current to drive the force

Amps need larger wires, volts need more insulation.  Insulation costs less than metal.

Actually you need at least 15 volts to charge a car battery.

If you have a large array, say 2000 watts, a lower voltage means a very high current, which means very heavy wire, which is expensive, and bulky.

in the example, 2000 watts at 12 volts is 170 amps. This requires a very thick cable. If you have to run it 50 feet, and lose only 1 volt, then you need resistance less than 0.006 ohms which is 0000 wire. Very expensive.

But this size installation would usually be connected for 96 volts or higher, which is a current of 21 amps, which is a much smaller wire size. 21 amps would require #7 wire, a lot cheaper.

Even at lower power, the wire size is still a significant part of the equation.

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Bill is right, you need about 15 working volts to charge a car battery, and the panel that you would connect in such an application would generally have an open-circuit voltage of 18 volts.

Solar cells are roughly constant-current devices at a given illumination, so if you have an 18-volt panel at 1 amp, when directly connected to a lead-acid battery, it will charge at 14-15 volts, 1 amp.  If you use a 36-volt, 1 amp panel, it will charge the same battery at 14-15 volts, 1 amp.  Basically, the latter setup would be a waste.

When you get to larger systems, you would use a switching charge controller that would allow you to take advantage of the extra voltage.  However, the charge controller itself consumes power, and this kind of arrangement only makes sense when the array is several hundred watts or more (and at that point, the battery bank is usually larger than a single car battery).