Normally I would say to buy them online direct from the lift company. This is more convenient and you get exactly what you want, particularly with regard to family discounts etc. One tour operator once suggested that we buy a 7 day lift pass through them and reccomended that we went to the lift company and buy an extra day for ourselves. The rep appeared confused when we pointed out that a 7 day plus an extra day was more expensive than an 8 day pass.
However, with the recent crash of the pound we noticed that in January the tour operator was charging for lift passes in pounds calculated at a preseason rate and saved about £15 per pass by buying through them. But we still had to go to the lift pass office to buy the extra 1/2 day passes!
Have a calculator with you as well as the currrent exchange rates and do your own sums. On a financial level it is not a difficult decision.
Now that lift passes, by and large, no longer require photographs there are machines that will issue them at the lift offices. The days of massive queues appear to be over. However, I still buy/renew my pass over the internet so I do not have to call into the office at all.