Go with the 500 volt.
Devices SHOULD be designed with some surge suppression built in and the lightning arrestors take that into account. They have to be rated high enough to not trip all the time from normal device operation.
My ham radios have a surge diode in them, it is rated at X voltage and has to be high enough that close in signals don't trip it and cause signal distortion, and it doesn't trip on transmit and distort the outgoing signal. Then I add on lightning snubber devices to protect against very high voltages and I have to rate them high enough to withstand the voltages present on my high power amplifiers outputs. So most won't kick in until 1,000 volts is reached!
In a solar system your DC lines can and will pick up incidental AC pulses from nearby radio transmitters, power lines etc. You have more on that cable than just the DC from the panels. That is why the higher voltage ratings, you don't want them tripping and dropping your solar output. A thunderstorm 5 miles away can induce 400+ volts on a buried cable for example. Not close enough for a near strike but you are still seeing effects from it as the ground potential rises and falls with the electrical energy in the storm.
