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Sure it's a tradeoff. But also consider, like the eiffel tower, both are large architectural installations with beauty one can marvel from a distance. No ticket needed. Haven't had your fill from the outside views, and want a deeper look, you'll probably go back home/hotel and look it up, learn a bunch about Gaudi on wikipedia, find out you can book tickets online. Get them, a couple of days later you can go back and explore the inside, and find that everyone there seems to be a little knowledgeable about Gaudi, and there are no lines, since there are no walk-ins.