Tap – if I know where it's come from; and I don't mean the clouds and 10,000 years percolating through the rocks of Derbyshire.
Bottle – if I'm thirsty and it's easier.
For example. I could never bring myself to fill a bottle from the public toilets at a service station, so I buy a bottle.
I have and would again drink straight from a stream (did it once in the Lakes only to find a dead sheep in it about 300 metres higher up the hillside).