There's (at least one) order of magnitude difference in popularity between the two. I don't think the average American could name the Sagrada Familia from a picture of it, for instance. For me, when I travel, sometimes I don't really look up what there is to see/do in a city until I'm there. When I landed in Barcelona, I didn't know what the Sagrada Familia was.
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.
This is because the Sagrada Familia is still way from being finished.
It was supposed to be an atonement temple relying on donations of sinners for progress but unfortunately there were not enough sinners in the Barcelona area or these were tight with their money.
Once the huge dome is finished, I’m sure it will play in the same league as ND in Paris.
I didn't know about this Cathedral until I was looking into what I might find in Barcelona earlier this year. It's entirely easy to imagine not knowing about it until I showed up.
