Liquid soap for sinks, because it's easier to press a plunger and get some soap than it is to pick up the bar, rub it around, put it back down again, deal with the little pool of water that forms under the soap and eats away at it...
Bar soap for showers, because when you use liquid soap, the water starts washing it away before you get to use it. For places where I don't want to rub a bar around (like my face) I roll the bar in my hands until I've worked up a lather, then spread it around.
In any shower that's not my own (be it a gym, pool, friend's house), I bring my own little bar of soap, as I don't like the idea of sharing the bar. Failing that, I bring a little bottle of liquid soap or body wash.