I suspect that actual technology results in the apparently paradoxical situation that is easier to visit the moon rather than a comfortable planet able to sustain life. I am obviously supposing the crew must return to Earth.
Am I correct thinking that if there would be a copy of Earth at the place of the Moon not manned missions would have been yet possible? This seems obvious but is rarely realised by people in general.
Similar (just similar) considerations apply to manned mission to Mars, which are getting more and more discussed in divulgative TV channels but seems to me almost totally fictional, at the current stage.
(I consider the various energy requirements for getting there, braking, descending, and back, and life sustaining stuff such as food, water and breathable gas. While the design of Apollo did it for the Moon, a two ways trip to a Earth like moon or Mars differ by many points and seems to me currently impossible.
Please note that I am aware that replacing moon with an Earth would change the solar system and perhaps a binary planet system orbiting a star is even physically impossible. I am concerned with distance and escaping speed only).
Edit: I would like to stress that I could have place two earth masses of concrete in my question instead of a couple of earths. Atmosphere is important in the sense that there are different braking mechanisms / dragging etc. There is not much emphasis in my Q about habitability, except that I have mentioned that counter-intuitively is easier to go to the moon than to some kind of a big planet supposed to have a breathable atmosphere. Just to avoid debate not inherent here.