If you don't need the height, then go with the 28" wide by 18" tall. Your back will thank you. The wider diameter will give you more efficient loading space, too. Just remember that you're going to lose about 1.5" due to the shelf you'll put on the bottom. As for the firing, because your kiln will have zone control it won't matter how tall it is.
I'd be happy to give you pricing if you don't have a local L&L distributor that you're working with.