I would also recommend buying it through the concierge as I followed sc flier's advice and avoided the queues last time we went, however, I booked friends into the hotel in February and the concierge told them that they would be unable to get a ticket for them but could assist in booking them online for them.

You can book tickets to the summit online - there is a little link which changes the prices to display the summit ticket prices - Eur13.10 per adult (as opposed to Eur8.10 for the 2nd floor).