What is the difference between "it's up to you" and "it's down to you"?

I see both "It's up to you" and "It's down to you" in conversations. So what's the difference?

Solution 1:

I felt "It's up to you" means that you're the one responsible for the job, "It's down to you" means that nobody else is left who can do the job except you.

ADDING ON: I realized from the comment that what I wanted to imply wasn't completely clear. As ngmiceli says, "down to you" suggests that there were others who could do the job, but for some reason, they are not available to do so (because they may be dead/busy/out to lunch), leaving only you.

Solution 2:

'It's up to you' connotes some element of personal choice. 'It's down to you' has more of an implication of responsibility or duty.

Solution 3:

1. Up to you = Responsible for a decision
2. Down to you = Responsible for an action

Examples

1. What happens next is up to you
2. The choice is yours, it's entirely up to you
3. It's now down to you to impress the judges
4. Everything that has gone wrong in my life is entirely down to you, you useless...

Solution 4:

Though the two tend to have the same meaning, I hear them used in very different contexts.

"It's up to you!" This often used in a more generic context, where one often wishes to encourage someone who holds the key to success in whatever endeavor is being talked about. It should also be noted that this phrase could simply imply, "the choice is yours."

On the other hand,"it's down to you" doesn't strike me personally as placing blame, though I certainly can see it being used that way. The first context that comes to my mind is a situation's success still rests in "your" hands, but this implies that there was some elimination that brought the scenario to this point. For example, a group of four people are all trying to best the current chess champion. The first three each go up against him and are defeated. Finally, one says to the fourth man, "It's down to you".

I don't see "it's down to you" meaning "it's your choice"; that feels somewhat unnatural to me, unless everyone else opted not to have an opinion in which case you were the only one left to make a choice.