Tip for the top of Eiffel Tower

My friend and I are visiting Paris at the end of July, and we'd really like to go to the top of the Eiffel Tower. However, tickets to the summit are already sold out, and only tickets for the second floor are still available.

Would it be better to buy the second-floor tickets online now to make sure we can at least go up there, or should we wait and try to buy summit tickets at the Eiffel Tower on the day of our visit by queuing at the ticket office?

Thanks in advance for any answer!