The gravity well of Mars is about ⅓ that of Earth. Humans won't fare well there. Also Mars' atmosphere used to be much thicker from what I understand. It is slowly losing its atmosphere due to lack of gravity.

Venus is easier to terradorm just because of it having a gravity well that is closer to 0.95-0.98 that of Earth. Throw some ice asteroids at it to create oceans and it may have an equal mass to the Earth.