Neither planet is at all hospitable "in the open air". They are both complete deserts with atmospheres that would kill us.
Venus is too close to the Sun so far too hot and heavily irradiated. It cannot possibly be called "A Failed Earth". Mars might be slightly nearer that unfortunate but meaningless accolade.
Mars is very cold, with a very thin atmosphere of mainly carbon-dioxide. I don't think it's inherently any more "radioactive" than the Earth but its thin atmosphere would not give much protection from the Sun's UV and ionising radiation - though that might be mitigated by the greater distance from the star.
Venus can be ruled out completely, but even Mars is not really suitable for "colonising" however you define that. Short-term scientific expeditions there are one thing, though it might be hard to find anyone willing to go. Treating the planet as Earth's granny-annexe, as many of the Mars-colony proponents imagine, is a totally different proposition.
Mars is nothing like the Earth and cannot support any Earthly form of life. Anyone there would be confined to their accommodation, with only short outdoor excursions in full space-suits possible.
