I would say life existed on both planets but died out due to the imperfections of Venus (no plate tectonics, too close to the sun, no moon) and Mars (Too small to support a thick, protective atmosphere like Earth’s, two tiny moons which will eventually crash into the planet, too fat to provide the type of climate Earth does). The last remnant of life on those planets were best suited to survive the harsh conditions.
Between the two tho, Mars has the best promise of being terraformed and colonized as an outpost for astromining in the asteroid belt. Of course, this would be when humanity has actually become a civilized and advanced species.
