Yes. You should always go for advanced tickets for westminister abbey as they really are beneficial. To start with, you will find a good amount of discount on your tickets and it will come at a pocket friendly rate. With westminster abbey ticket you also get a guided tour which again is necessary as there is a lot to discover in the place and you don't want to miss out on anything. The booking process online is also pretty easy as opposed to physical booking which can sometimes get tiring and with all the enthusiasm and excitement that you enter into the place, by the time you have the tickets, they will wareoff. So, yes if you have an option to get online westminster abbey ticket go for it.