With the Falcon 9 soft landing there seem to be two main challenges:
- Getting it in the right place (with a very small margin of error, the barges aren't that huge)
- Getting it to stay upright
To my (very uneducated in this field) mind, the first bit seems much harder to achieve (not that I am claiming the upright bit is easy of course). Yet with both attempts so far, it has been spot on target. It is very impressive technically, and if the stages can genuinely be reused safely, it is very cool indeed.