I play on two servers, one for my ally characters and the other for my horde. On one server I can sell my thorium and adamantite ore for about the same price and on the other I can sell thorium for just a bit more. It's hard to say which is really the best, it depends on the price on your server and the demand for the ore. Try farming some of each and see how much you can get for each and then you'll see which is the best to farm for and sell on your server.