You might want to know that the TWSBI 580 is not designed to be posted, so you should keep the cap in your other hand, or stored elsewhere. You can post it, but then it is so long that it becomes head heavy and unbalanced. Also, if you turn the cap you might turn the piston down and spit ink into your papers.
The TWSBI Mini is designed to be posted. All the mentioned problems are avoided. Still I like the Mini Classic instead of the clear as that let me see the remaining ink while it hides any ink mess at the nib (while capped). It is also the perfect size for the shirt pocket.