Positive numbers begin with 0, negative numbers begin with 1. Negation is easy. But addition and subtraction are hard. One's complement . Positive numbers begin with 0, negative numbers begin with 1.
Negation is easy, but slightly time consuming.
It's not clear that addition and subtraction are any easier. Two's complement . To negate a number, flip the bits and then add 1, using standard addition. Throw away any overflow bits. Positive numbers begin with 0.