Goofyhoofy said:

The Airbus 380 has a passenger capacity 15%-20% larger than the Dreamliner. Airlines will balance that against the Dreamliner's 20% fuel savings (along with other factors, like legacy fleets, obviously).

The A380 passenger capacity is waaay bigger than 15-20% of a 787. According to wikipedia:

B787-8 (three cabins): 210
B787-9 (three cabins): 250-290

A380-800 capacity (three cabins): 525
A380-800 capacity (two cabins): 644
A380-800 capacity (one cabins): 853

Whenever I see an A380 in the air, but near the ground I can't help but gape...those things are so freaking big I am mystified that they can fly!

In terms of pricing a 787 is not quite half the price of an A380:

A380-800: $390 million

B787-8: $207 million
B787-9: $244 million

Obviously these are list prices but it gives us an idea about relative fixed costs. I think the 787's role is intended to compete in the space that the A330 and A340 currently occupies.

In terms of demand for the aircraft, the A380 has a lifetime total number of orders of 262 (thus far) and has been in operation since 2008.

On the other hand, the 787 started service late 2011 but has already amassed about 850 orders.

At a ratio of about 3.25:1 in favor of the 787, I think the airlines have been pretty definitive about which plane they want.