I see both "It's up to you" and "It's down to you" in conversations. So what's the difference?
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.
Though the two tend to have the same meaning, I hear them used in very different contexts.
"It's up to you!" This is 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 beat 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.
I would personally interpret It's up to you to be referring to a choice.
You can have cake or you can have ice cream. It's up to you.
It's down to you seems to refer more to an obligation
This project has to be finished by 5pm, its down to you to get it done.
If this seems like a weak attempt to understand the phrases, try reversing them. They sound very strange.
You can have cake or you can have ice cream. It's down to you.
This project has to be finished by 5pm, its up to you to get it done.
"It's up to you" implies (at least for me) initiative and a certain level of faith in 'you'. As in, there is a problem, and it's up to you to fix it, since you are the best person for the job, it's your job to fix it, you have the tools to fix it, etc. I don't know if the two phrases are technically linked, but I see a parallel with "I'm up for it".
"It's down to you" is a little more negative, implying that the problem was your fault, or that the reason you're being relied on to fix it is simply because there's no one else to do it.
There's also the other meaning of "it's up to you", which is what Barrie said: the choice is yours. In this case, "it's down to you" implies generally the same thing, but I see it as carrying the extra meaning that everyone else has decided, and now it's time for you to make your choice (everyone else has gone, you're the only one left-- it's down to one, which is you).
The service is not processing inputs correctly so ignore the other post and let's try again here.
Both are mere idiomatic expressions. Up to you connotes an obligation and its tone is typically affirmative. Down to you, connotes an obligation, or usurpation (neither necessarily affirmative) of last, negative, deviant, or derelict resort. The expression, down to me, first came into (more less than more) vogue in the 1970s as a refrain in the lyric of the rock song, "Under My Thumb," sung by Mick Jagger, Rolling stones: also the song in progress at the Altamont Pass (California) concert, during which a killing perpetrated by individuals in the hell's angels motorcyle club took place.
Down to, as an alteration of up to, implies not fault or culpability but, rather, the absence or exhaustion, for good or ill, of all alternative choices. It's "down to you" because no one else was or remains available, as a matter of due course or of choice.
- be down to somebody
if an action or decision is down to you, it is your responsibility:
It's down to me to make sure that everyone is happy.
- be up to somebody (link)
a) used to say that someone can decide about something:
You can pay weekly or monthly - it's up to you.
b) used to say that someone is responsible for a particular duty:
It's up to the travel companies to warn customers of any possible dangers.
In the following gap fill exercise, Cambridge accepts both versions in their First certificate (level B2), Paper 3, Use of English 2008.
Your entry can take any form—a piece of writing, a picture, or even architectural plans. It is completely _________ to you.
Instinctively, I'd prefer up to you because candidates have the maximum freedom of choice; however, I can also see how down to you would imply they alone, and no one else, are responsible for that choice.