This is a frequently asked question by tourists coming to Barcelona. My recommendation is to buy the Entry + Audio Guide + Towers option for 29 euros.
I have reviewed all options for tickets listed on the Sagrada Familia official site:
Basic entry ticket to Sagrada Familia for 15,00€
This option is good for those in a hurry and want to photograph the facade and interiors of Sagrada Familia. %%A basic entry ticket also works for those who are happy to see Sagrada Familia using the free downloadable guide.
Entry + Sagrada Familia Audio Guide for 22,00€
Sagrada Familia entry with audio guide, is the most popular option. Audio guide is strongly recommended to discover the vision of Gaudi in all the details hidden in the architecture and sculptures. If you are traveling as a couple, you can share one audio guide, provided you use your own earphone.
Guided tour for 24,00€
If you are lucky, you will be assigned a good guide who will narrate many inside stories and tips on Sagrada Familia. My recommendation is to choose the Sagrada Familia audio-guided tour over the official-guided tour.
This ticket option just adds the entry to Gaudi's museum at Parc Guell. I do not recommend this option, as explained in this post
Entry + audio-guide + towers for 29,00€ - Buy this!
The 7 extra euros you pay in this ticket to climb the towers of Sagrada Familia is definitely worth the price. When you book the ticket, you will be asked to select the tower to climb. My vote is for Nativity Tower as explained in this post.
This (entry + audio guide + towers) is my top recommended ticketing option for Sagrada Familia. However, if you have a fear of heights, claustrophobia, or a mobility issue that can prevent you from climbing the towers of Sagrada Familia, skip this option.
Normally tickets to La Sagrada Familia are released online a month in advance.
TIP - Skip the Sagrada Familia towers and go with Option 2 (entry + audio guide) for 22€.
This way you can see the important parts of Sagrada Familia (exterior and interior) and still save 7€ per ticket.
Use that 7€ saving to enter the paid part of Parc Guell, the Monumental Zone, instead.