In this case, 1000V@1A is more harmful, with of a sufficient current level to be fatal in some cases. The 1000A@1V situation wouldn't be as harmful because the body's resistance (anywhere from 1,000 to 100,000 ohms) would prevent that much current from flowing. Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance between them. So in practice you wouldn't get anywhere near 1000A flowing due to the human body's resistance.