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Both are powerful green aliens with strange powers that have dedicated their lives to defending the earth.

Who will prevail in a fight?

Well there's always wikipedia for info.

Well i think Martian Manhunter wins. Superman says that he's almost on the same power level as he is, and also states that he's more powerful than the rest of the JLA combined in a recent issue of Superman-Batman (Supergirl arch i think). So we're talking about a guy who not only has superman class strength, (proven to be stronger than Goku), but is also a telepath. No major weaknesses except fire (which piccolo is unaware of) i'm guessing he wins.