If the option is available, we prefer to buy tickets online. Most of the time it's a cheaper option, and saves some time. However, when there is a ridiculous "handling" fee to be paid online, then we don't mind mind queuing up at the kiosk. Also, if we are double minded about going to an event, then we will wait till the day of the event to re-evaluate before buying the tickets.
I don't stick to just one of the other; I'll do both. It usually depends on cost and convenience. Which way is cheaper? Will I have to go out of my way to get them at the box office? Will I still get in early enough if I buy on the day?