The real question is what you want the screen to do. I think many people have not been happy with the summer or roadster screen. It seems to cause a lot of buffeting on the head. If you want really want good protection Then the Parabellum Sport Shield seems to give very good protection but it is pretty tall. It is also cheaper than the Triumph labeled screens. You also get a 30 day trial to see if you like it.
http://www.parabellum.com/?&DID=22&Filter=Browsing
Many people like the Dart but it will provide less protection. Looking to see more feedback on that one.