What would you recommend, Park Guel or la Pedrera? I'm trying to decide between the two. Also does anyone know the websites to purchase these tickets in advance?
By La Pedrera I think you mean Casa Mila, the apartment block. They are very different (one is a park, the other an apartment building). While I think Parc Guell is really nice, I'd suggest the apartment building. UNLESS... you are also visiting Casa Batllo, his other famous apartment building. I actually think that is a better one to visit if you are only seeing one of the two (they are 3 blocks from each other). So if you are seeing Batllo, then go to Parc Guell.
Parc Guell is not easy to get to. Metro ride and then a long uphill walk (with some escalators). And not very nice if it's raining or super hot. You can also take a taxi.
And I assume you will be visiting Sagrada Familia.
When they fenced off the main area of parc guel and started charging admission the parc has become even more jammed packed and, in my opinion, lost what little charm it once possessed. The issue is the number of vendors hawking their wares and since folks are paying to attend they seem more inclined to just hang around. I would do both buildings.
Thank you all very much! We look forward to seeing as much as possible!