We use a mixture of both. For recipes, tsp, tbsp, cups, etc..., but I get confused with pints, quarts, etc... We'll say 250 ml, 500 ml or a litre. Gas in litres. Temp in celcius (I have a hard time converting to farenheit, thank God for online conversion calc.). Speed or driving distance are in Kilometers. Length are both...both centimers, meters, inches and yards. Weight is most often in pounds, but depending on what it is. Most of us calculate our weight in pounds, but you'd buy a kilo of sugar and a 10 pound bag of potatoes.
In other words, it's a total mish-mash of both!