Are you wondering about the Severin, or the Severin Medium?
I guess my vote will depend on how experienced you are with plugs. As Naughty Student says, the thinner the neck, the more comfortable for long term wear. I use my Pure Plug most often when I want something long-term.
That said, I've got a Severin Medium which wouldn't be impossible to use for long-term wear, even though its neck is thicker than the Severin Small.
How experienced are you? I'm tempted to vote for the Screw because it's pretty and I'm personally curious about it... but I really enjoy my Severin too. Either would be a good choice provided you're ready for the girth.