That depends. The sandstone corners make it sort of blend into the desert background, while the nether brick make it look more menacing and it also stands out.
Personally I think sandstone corners would be better, maybe if your going to have some netherblock on the inside you could use the nether brick corners..
Here is a photo of the interior, when I'm on the ground: http://screensnapr.com/v/nJ76UN.jpg
Here is a birds eye view of the interior: http://screensnapr.com/v/A69p8x.jpg
As you can see, (http://screensnapr.com/v/g8uTK4.jpg) the sides are all open space.
What can I put there to fill the space up?
I think I'm going with Nether Brick.