Neither. Obsidian and Coal look terrible as primary blocks... I assumed everyone would know this.. :/
My suggestion would be to use the standard netherbrick/stonebrick style and slightly up the ratio of netherbrick. The evil effect should be acheived by the shape of the build (like imposing walls, tall warped towers, claw like battlements etc)

Coal and obby can be used VERY sparingly as decoration.

This is just my opinion of course. Feel free to check out my entry for inspiration.
http://www.planetminecraft.com/project/ ... um-regnum/

It is castleless as well, but ill be adding one shortly.
Tell me what you think. (there are 0 blocks of obsidian and coal in the map)

Obsidian and coal AND nether bricks will make it look great

Obsidian, because coal blocks catch fire easily.

I like both.
Then again, I like all the textures on blocks.

Coal block. I hate the obsidian texture.

none, both would look horrible for a castle. You shhould use more variety of blocks

Obbsidian looks creepier, but coal block is blacker. u decide.

I vote obby

obsidian