For the singular purpose of human habitation, not really. Unless we find significant resources on other moons or planets and their value outweighs the cost of extraction, even with better space travel and hauling, the danger of living on extraterrestrial bodies remains. Space is dangerous even with the best technology - our science fiction even includes this quite often where breaches to spacecraft or colony buildings leads to disaster.

That being said if research was the main goal and we could justify the cost of it then the icy moons of Saturn and Jupiter, notably Titan, Enceladus and Europa would be the best targets if you care about discovering life as they are most likely to harbor it.

For your biology question re gravity. There are actually a lot of interesting studies on this using the International Space Station. Sometimes when astronauts return to Earth they cannot stand at all due to their muscles degrading as they become disused. For bones I would think that without as much gravity they would be able to grow longer resulting in taller people. Indeed, when astronauts return from the ISS they are taller than when they left as their spine is less compressed.

This also has a lot to do with distance - relatively speaking Mars and our own Moon are very close to Earth. The moons of Jupiter and Saturn are incredibly far and the logistics of keeping people alive on those moons is beyond our current capabilities.

Even a Mars colony - a true one with humans constantly inhabiting it - would be incredibly difficult. If anything goes wrong, human lives will be lost without question. It is more likely that a Martian base would be first established as a research outpost, only housing humans on explicitly research-focused missions.

Source: I'm an astrobiology PhD (fifth year) at McMaster University.