Are solar cells more efficient on Mars or on Earth?

Mars receives less energy per square meter but less is absorbed by the atmosphere so the solar energy per square foot at Mars equator is about the same as say in Toronto Canada. However, in Toronto, you could tilt the solar panel towards the Sun to receive more energy. Of course, the One on Earth would always work better because someone can clean the panels. On Mars, we often have to wait for the Martian winds to clear the panels enough to continue functioning.

Because of the increased distance from the sun, solar cells are almost useless on Mars. Mars is about twice as far from the Sun as the Earth, so in accordance with the inverse square law, receives about a quarter of the solar energy the Earth does. The main reason most Martian rovers are tiny is because they're solar powered, and have to be extremely light so the underpowered motors to be able to move them. NASA primarily uses solar power because it's cheaper than nuclear powered, and less controversial. Curiosity is nuclear powered, and is about the size of a Minicouper. Any further out than Mars, and solar panels aren't worth the weight.

Earth, being closer to the Sun.

Mars may not seem to have issues like cloud cover, but in fact have to deal with far worse in the form of dust storms, some that go global and last for weeks or months. And as some of the rovers have found can totally cover the panels in dust unless something comes along (such as a dust devil) to blow the dust off again.

Mars being more distant from Sun gets less solar radiation so power output will be less for the same panel.It is not efficiency problem but power input is less.

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Oddly enough, current solar cells would be more efficient on the moon.

They would work great on Mercury, not so much on Venus (too thick atmosphere), okay on Earth, not so great on Mars. Distance.

Well the one on earth because there is people to service it.