If something is up to somebody it means that person has a responsibility to take care of it, or make a decision. Nobody else will (or can) do it:
"It's up to her to fix this mess."
"I'm just driving, it's up to you where we go!"
You can use this phrase for saying that someone has a choice:
This meal comes with a side of either salad or potato waffles - it's up to you
In a way it's the same idea - you are the one who makes the decision - but it sounds more like a positive thing than a responsibility. But it's still your decision to make! It's up to you (to decide what you're going to have).
When X is down to Y, it usually means that Y is the cause of or reason for X. It's similar to the phrases it comes down to or boils down to - you're talking about the basis, the root of the issue. It's like getting to the bottom of something. So when you say something is down to someone, it means they're responsible for it (whether it's good or bad):
All this success is down to your hard work this year!
The damage is all down to that storm
The difference is that up to implies an action that needs to be taken, and who or what will be responsible for the outcome. Down to is used for the results of that outcome, the consequences of something, and to describe who or what is responsible for the resulting situation. They're used in different contexts - are we talking about something that will happen in the future, or something that already happened in the past?
Sometimes people will use down to in the up to sense, often when other people who could have taken responsibility no longer can, so there's nobody else. "It all falls down to you." (This is possibly a variation on falls to, and you can see how a down could end up added to falls!)
Either way, it should be clear from the context what they actually mean. Are we talking about cause, or effect?