General - the problem with Mars is that it's atmosphere is only 2% of the density of Earth, so even if we could terraform it, nothing above a simple moss would be able to live there.

Venus on the other hand, while being more of a challenge, does have a higher chance. It's got a good (if rather overheated atmosphere) and is the same size as Earth! And how is it more of a challenge? Because it has no hydrogen!

Face it - unless we wear suits while outside and live in pressurised bubble cities, the colonisation of other planets is impossible. Unless we do somehow come up with interstellar travel and find another planet similar to Earth, we're doomed. Doomed i tells ya.