Casa Mila or Casa Batlo?

I'll answer by explaining the difference and letting you choose. Casa Mila has an excellent museum of Gaudi's work in the attic. The attic space is really cool with brick vaults. It's a nice museum and explains well how he came up with some of his designs. Sagrada Familia also has a small museum that repeats a little of the design philosophy. You tour a furnished apartment, which is pretty and of an Art Nouveau style. It gives you a sense of how an upper middle class family lived at the turn of the 20th century. Lastly, you tour the roof, which is a wonderscape of his designs and forms. Sometimes there is an art exhibit in the main floor gallery, which I think comes included with the ticket price (not sure so don't quote me). Casa Batllo has a tour of an unfurnished apartment that is over-the-top of his design. Think "10,000 Leagues Under the Sea" fantasy. You'll see the interior light court, which is beautifully tiled and fun to see. You walk through the attic space, which was more of the vaults like at Milo but not as grand or beautiful (it was the laundry and servant spaces). The roof garden is nice with some great, colorful tiled sculptures but not as elaborate as Milo. You won't go wrong with either choice. Both are pricey tickets.

Thank you so much for the reply,Douglas. It is very helpful. chun

Both Gaudi buildings are worth seeing. However, do you plan on missing the Sagrada Familia? Truly stunning outside and inside.

I've only seen the outside of Casa Batlo, but we chose to go inside Casa Mila primarily because the chimneys fascinated me. So cool! I was quite happy with our pick because they also have displays and explanations of Gaudi's other works. Have fun in Barcelona =)