The leather Smart Cover is available in four colors: Saddle Brow, Taupe, Midnight Blue and Black.
The polyurethane Smart Cover is available in seven colors: Mist Blue, Pollen, Flamingo, White, Pink Sand, Midnight Blue and Charcoal Grey.
Personally, I prefer the polyurethane as it is cheaper and easier to maintain than the leather counterpart. I am speaking based on my experience with the leather case of my iPhone 6s Plus. That is the reason why I switched to silicone case for my iPhone 7 Plus.
For iPad Air and iPad Air 2, Apple has Smart Cases for them, and Smart Cases make sense because it protects both the screen and the back.
For the new iPad and iPad Pro, there are no Smart Cases. To protect the screen, you need to get the Smart Cover, and if you want to protect the back, you need to get a separate Silicone Case from Apple.
There is no Silicone Case available for the 12.9″ and 10.5″ iPad Pro. Only the 9.7″ iPad Pro has it and it retails for S$98 in 16 different colors.
I hope both the 12.9″ and 10.5″ iPad Pro will have Smart Cases and Silicone Cases eventually.