On, unless you have a particularly tricky situation in which to get moving. Sometimes gunning it is a valid tactic, and if you can’t you might be in trouble. However once you have got yourself out of your snowdrift or hilly driveway or whatever and are on the open road, leave it on.
One criticism of the MkII Prius from people living in snowy countries is that you can’t turn off traction control or the ESP.
However the traction control on the Passat allows a fair bit of slippage, which is a little less than useful on snow sometimes. The Prius TC allows zero slippage which is actually quite good if you are trying to find a sliver of traction. It’s when there isn’t that it causes problems.