They mean the same thing, at least in American English. It is a strange quirk of English slang that to be "up for [something]" and to be "down for [something]" both mean to be "interested/available to do [something]."
I've always interpreted "up" as coming from its meaning "ready, available" and "down" from its use in expressions like "put my name down" i.e. write down that I'm interested.
Andrew is correct. In US slang they mean the same thing.
To be „up for something“ is much older. It`s not archiac, but something you would have heard in the 1970s and 80s, as well as earlier. To be „down“ for something is of fairly recent vintage. The people I hear saying „down for“ are those who were teens in the late 90s and beyond. I don’t know when the change exactly occurred but I never heard anyone say „down for“ something in the 1980s or earlier. I don’t know if the expression, „down for“ is still being used by teenagers today, or if it`s been replaced by something else. But think of it as a generation split. People over 40 tend to say „up for“ while those under 40 might say „down for“ but they are the same.
