As for visiting the interior, I can only speak to Casa Batllo, as we ran out of time to visit the interior of Casa Mila (although our tour guide highly recommended it, and apparently just the roof alone is worth the price of admission).
We really enjoyed our tour of the Casa Batllo interior. When you arrive (after you, once again, BUY YOUR ENTRY TICKET ONLINE TO AVOID THE LINE!), you get an audio guide in your language.
But wait!
This is NOT your average audio guide!
It’s an augmented reality video guide.
In other words, you plug in your big blue headphones (they actually complement the interior of the house rather nicely), and hold up an iPhone-like device as you walk through the house.
When you hold it up to an otherwise empty room, you can see a visual representation of what the room would have looked like with furnishings, or maybe what Gaudi imagined as he designed it (for instance, if you hold it up to some of the windows, you can watch the window on your screen morph into the sea creature on which it was based. Then you watch it swim away, leaving you with the original window). It is whimsical and fascinating. It’s probably the only time you’ll ever touch a banister inspired by a spine or stand in a room inspired by a ribcage. Oh, and the views from the roof (which, by the way, is a dragon) are gorgeous.